feat: add mvp language server (#8515)

Resolves #8400

38 files changed, 4878 insertions, 47 deletions

diff --git a/Cargo.lock b/Cargo.lock
index a34c10ed9..224d0f322 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -366,11 +366,21 @@ version = "0.4.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b153fe7cbef478c567df0f972e02e6d736db11affe43dfc9c56a9374d1adfb87"
 dependencies = [
- "crossbeam-utils",
+ "crossbeam-utils 0.7.2",
  "maybe-uninit",
 ]
 
 [[package]]
+name = "crossbeam-channel"
+version = "0.5.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "dca26ee1f8d361640700bde38b2c37d8c22b3ce2d360e1fc1c74ea4b0aa7d775"
+dependencies = [
+ "cfg-if 1.0.0",
+ "crossbeam-utils 0.8.1",
+]
+
+[[package]]
 name = "crossbeam-utils"
 version = "0.7.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -382,6 +392,17 @@ dependencies = [
 ]
 
 [[package]]
+name = "crossbeam-utils"
+version = "0.8.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "02d96d1e189ef58269ebe5b97953da3274d83a93af647c2ddd6f9dab28cedb8d"
+dependencies = [
+ "autocfg 1.0.1",
+ "cfg-if 1.0.0",
+ "lazy_static",
+]
+
+[[package]]
 name = "darling"
 version = "0.10.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -437,6 +458,7 @@ dependencies = [
  "bytes 0.5.6",
  "chrono",
  "clap",
+ "crossbeam-channel 0.5.0",
  "deno_core",
  "deno_crypto",
  "deno_doc",
@@ -457,9 +479,12 @@ dependencies = [
  "lazy_static",
  "libc",
  "log",
+ "lsp-server",
+ "lsp-types",
  "nix",
  "notify",
  "os_pipe",
+ "percent-encoding",
  "regex",
  "ring",
  "rustyline",
@@ -1315,6 +1340,32 @@ dependencies = [
 ]
 
 [[package]]
+name = "lsp-server"
+version = "0.5.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "69b18dfe0e4a380b872aa79d8e0ee6c3d7a9682466e84b83ad807c88b3545f79"
+dependencies = [
+ "crossbeam-channel 0.5.0",
+ "log",
+ "serde",
+ "serde_json",
+]
+
+[[package]]
+name = "lsp-types"
+version = "0.84.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3b95be71fe205e44de754185bcf86447b65813ce1ceb298f8d3793ade5fff08d"
+dependencies = [
+ "base64 0.12.3",
+ "bitflags",
+ "serde",
+ "serde_json",
+ "serde_repr",
+ "url",
+]
+
+[[package]]
 name = "matches"
 version = "0.1.8"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -1513,7 +1564,7 @@ checksum = "77d03607cf88b4b160ba0e9ed425fff3cee3b55ac813f0c685b3a3772da37d0e"
 dependencies = [
  "anymap",
  "bitflags",
- "crossbeam-channel",
+ "crossbeam-channel 0.4.4",
  "filetime",
  "fsevent",
  "fsevent-sys",
@@ -2283,6 +2334,17 @@ dependencies = [
 ]
 
 [[package]]
+name = "serde_repr"
+version = "0.1.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2dc6b7951b17b051f3210b063f12cc17320e2fe30ae05b0fe2a3abb068551c76"
+dependencies = [
+ "proc-macro2 1.0.24",
+ "quote 1.0.7",
+ "syn 1.0.48",
+]
+
+[[package]]
 name = "serde_urlencoded"
 version = "0.6.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
diff --git a/cli/Cargo.toml b/cli/Cargo.toml
index a8d6e9c2b..95ffac7fe 100644
--- a/cli/Cargo.toml
+++ b/cli/Cargo.toml
@@ -32,22 +32,24 @@ winres = "0.1.11"
 winapi = "0.3.9"
 
 [dependencies]
-deno_crypto = { path = "../op_crates/crypto", version = "0.3.0" }
 deno_core = { path = "../core", version = "0.69.0" }
+deno_crypto = { path = "../op_crates/crypto", version = "0.3.0" }
 deno_doc = "0.1.17"
+deno_fetch = { path = "../op_crates/fetch", version = "0.12.0" }
 deno_lint = "0.2.12"
 deno_web = { path = "../op_crates/web", version = "0.20.0" }
-deno_fetch = { path = "../op_crates/fetch", version = "0.12.0" }
 
 atty = "0.2.14"
 base64 = "0.12.3"
 bytes = "0.5.6"
 byteorder = "1.3.4"
 clap = "2.33.3"
+crossbeam-channel = "0.5.0"
 dissimilar = "1.0.2"
 dlopen = "0.1.8"
-encoding_rs = "0.8.24"
 dprint-plugin-typescript = "0.35.0"
+encoding_rs = "0.8.24"
+env_logger = "0.7.1"
 filetime = "0.2.12"
 http = "0.2.1"
 indexmap = "1.6.0"
@@ -55,31 +57,33 @@ jsonc-parser = "0.14.0"
 lazy_static = "1.4.0"
 libc = "0.2.77"
 log = "0.4.11"
-env_logger = "0.7.1"
+lsp-server = "0.5.0"
+lsp-types = { version = "0.84.0", features = ["proposed"] }
 notify = "5.0.0-pre.3"
+percent-encoding = "2.1.0"
 regex = "1.3.9"
 ring = "0.16.15"
 rustyline = { version = "7.0.0", default-features = false }
 rustyline-derive = "0.4.0"
+semver-parser = "0.9.0"
 serde = { version = "1.0.116", features = ["derive"] }
 shell-escape = "0.1.5"
-sys-info = "0.7.0"
 sourcemap = "6.0.1"
 swc_bundler = "0.17.5"
 swc_common = { version = "0.10.6", features = ["sourcemap"] }
 swc_ecmascript = { version = "0.14.4", features = ["codegen", "dep_graph", "parser", "react", "transforms", "visit"] }
+sys-info = "0.7.0"
 tempfile = "3.1.0"
 termcolor = "1.1.0"
 tokio = { version = "0.2.22", features = ["full"] }
 tokio-rustls = "0.14.1"
 # Keep in-sync with warp.
 tokio-tungstenite = "0.11.0"
-webpki = "0.21.3"
-webpki-roots = "=0.19.0" # Pinned to v0.19.0 to match 'reqwest'.
+uuid = { version = "0.8.1", features = ["v4"] }
 walkdir = "2.3.1"
 warp = { version = "0.2.5", features = ["tls"] }
-semver-parser = "0.9.0"
-uuid = { version = "0.8.1", features = ["v4"] }
+webpki = "0.21.3"
+webpki-roots = "=0.19.0" # Pinned to v0.19.0 to match 'reqwest'.
 
 [target.'cfg(windows)'.dependencies]
 winapi = { version = "0.3.9", features = ["knownfolders", "mswsock", "objbase", "shlobj", "tlhelp32", "winbase", "winerror", "winsock2"] }
diff --git a/cli/ast.rs b/cli/ast.rs
index 10d7b5383..ef6468310 100644
--- a/cli/ast.rs
+++ b/cli/ast.rs
@@ -354,7 +354,7 @@ impl ParsedModule {
   }
 }
 
-fn parse_with_source_map(
+pub fn parse_with_source_map(
   specifier: &str,
   source: &str,
   media_type: &MediaType,
diff --git a/cli/file_fetcher.rs b/cli/file_fetcher.rs
index 0d11852d1..5b2f6f74c 100644
--- a/cli/file_fetcher.rs
+++ b/cli/file_fetcher.rs
@@ -131,7 +131,7 @@ fn fetch_local(specifier: &ModuleSpecifier) -> Result<File, AnyError> {
 
 /// Given a vector of bytes and optionally a charset, decode the bytes to a
 /// string.
-fn get_source_from_bytes(
+pub fn get_source_from_bytes(
   bytes: Vec<u8>,
   maybe_charset: Option<String>,
 ) -> Result<String, AnyError> {
@@ -161,7 +161,7 @@ fn get_validated_scheme(
 
 /// Resolve a media type and optionally the charset from a module specifier and
 /// the value of a content type header.
-fn map_content_type(
+pub fn map_content_type(
   specifier: &ModuleSpecifier,
   maybe_content_type: Option<String>,
 ) -> (MediaType, Option<String>) {
diff --git a/cli/flags.rs b/cli/flags.rs
index 5ff21971d..2210d7565 100644
--- a/cli/flags.rs
+++ b/cli/flags.rs
@@ -17,6 +17,9 @@ pub enum DenoSubcommand {
     source_file: String,
     out_file: Option<PathBuf>,
   },
+  Cache {
+    files: Vec<String>,
+  },
   Compile {
     source_file: String,
     output: Option<PathBuf>,
@@ -35,9 +38,6 @@ pub enum DenoSubcommand {
     code: String,
     as_typescript: bool,
   },
-  Cache {
-    files: Vec<String>,
-  },
   Fmt {
     check: bool,
     files: Vec<PathBuf>,
@@ -54,6 +54,7 @@ pub enum DenoSubcommand {
     root: Option<PathBuf>,
     force: bool,
   },
+  LanguageServer,
   Lint {
     files: Vec<PathBuf>,
     ignore: Vec<PathBuf>,
@@ -293,6 +294,8 @@ pub fn flags_from_vec_safe(args: Vec<String>) -> clap::Result<Flags> {
     lint_parse(&mut flags, m);
   } else if let Some(m) = matches.subcommand_matches("compile") {
     compile_parse(&mut flags, m);
+  } else if let Some(m) = matches.subcommand_matches("lsp") {
+    language_server_parse(&mut flags, m);
   } else {
     repl_parse(&mut flags, &matches);
   }
@@ -349,6 +352,7 @@ If the flag is set, restrict these messages to errors.",
     .subcommand(fmt_subcommand())
     .subcommand(info_subcommand())
     .subcommand(install_subcommand())
+    .subcommand(language_server_subcommand())
     .subcommand(lint_subcommand())
     .subcommand(repl_subcommand())
     .subcommand(run_subcommand())
@@ -685,6 +689,10 @@ fn doc_parse(flags: &mut Flags, matches: &clap::ArgMatches) {
   };
 }
 
+fn language_server_parse(flags: &mut Flags, _matches: &clap::ArgMatches) {
+  flags.subcommand = DenoSubcommand::LanguageServer;
+}
+
 fn lint_parse(flags: &mut Flags, matches: &clap::ArgMatches) {
   let files = match matches.values_of("files") {
     Some(f) => f.map(PathBuf::from).collect(),
@@ -1076,6 +1084,18 @@ Show documentation for runtime built-ins:
     )
 }
 
+fn language_server_subcommand<'a, 'b>() -> App<'a, 'b> {
+  SubCommand::with_name("lsp")
+    .setting(AppSettings::Hidden)
+    .about("Start the language server")
+    .long_about(
+      r#"Start the Deno language server which will take input
+from stdin and provide output to stdout.
+  deno lsp
+"#,
+    )
+}
+
 fn lint_subcommand<'a, 'b>() -> App<'a, 'b> {
   SubCommand::with_name("lint")
     .about("Lint source files")
@@ -1953,6 +1973,18 @@ mod tests {
   }
 
   #[test]
+  fn language_server() {
+    let r = flags_from_vec_safe(svec!["deno", "lsp"]);
+    assert_eq!(
+      r.unwrap(),
+      Flags {
+        subcommand: DenoSubcommand::LanguageServer,
+        ..Flags::default()
+      }
+    );
+  }
+
+  #[test]
   fn lint() {
     let r = flags_from_vec_safe(svec![
       "deno",
diff --git a/cli/http_cache.rs b/cli/http_cache.rs
index 9cf2adc1a..dd5f4dc3f 100644
--- a/cli/http_cache.rs
+++ b/cli/http_cache.rs
@@ -72,7 +72,7 @@ pub fn url_to_filename(url: &Url) -> PathBuf {
   cache_filename
 }
 
-#[derive(Clone)]
+#[derive(Debug, Clone, Default)]
 pub struct HttpCache {
   pub location: PathBuf,
 }
diff --git a/cli/lsp/README.md b/cli/lsp/README.md
new file mode 100644
index 000000000..dcc953273
--- /dev/null
+++ b/cli/lsp/README.md
@@ -0,0 +1,23 @@
+# Deno Language Server
+
+The Deno Language Server provides a server implementation of the
+[Language Server Protocol](https://microsoft.github.io/language-server-protocol/)
+which is specifically tailored to provide a _Deno_ view of code. It is
+integrated into the command line and can be started via the `lsp` sub-command.
+
+> :warning: The Language Server is highly experimental and far from feature
+> complete.
+
+This document gives an overview of the structure of the language server.
+
+## Acknowledgement
+
+The structure of the language server was heavily influenced and adapted from
+[`rust-analyzer`](https://rust-analyzer.github.io/).
+
+## Structure
+
+When the language server is started, a `ServerState` instance is created which
+holds all the state of the language server, as well as provides the
+infrastructure for receiving and sending notifications and requests from a
+language server client.
diff --git a/cli/lsp/analysis.rs b/cli/lsp/analysis.rs
new file mode 100644
index 000000000..370b41c45
--- /dev/null
+++ b/cli/lsp/analysis.rs
@@ -0,0 +1,324 @@
+// Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
+
+use crate::ast;
+use crate::import_map::ImportMap;
+use crate::media_type::MediaType;
+use crate::module_graph::parse_deno_types;
+use crate::module_graph::parse_ts_reference;
+use crate::module_graph::TypeScriptReference;
+use crate::tools::lint::create_linter;
+
+use deno_core::error::AnyError;
+use deno_core::ModuleSpecifier;
+use deno_lint::rules;
+use lsp_types::Position;
+use lsp_types::Range;
+use std::cell::RefCell;
+use std::collections::HashMap;
+use std::rc::Rc;
+
+/// Category of self-generated diagnostic messages (those not coming from)
+/// TypeScript.
+pub enum Category {
+  /// A lint diagnostic, where the first element is the message.
+  Lint {
+    message: String,
+    code: String,
+    hint: Option<String>,
+  },
+}
+
+/// A structure to hold a reference to a diagnostic message.
+pub struct Reference {
+  category: Category,
+  range: Range,
+}
+
+fn as_lsp_range(range: &deno_lint::diagnostic::Range) -> Range {
+  Range {
+    start: Position {
+      line: (range.start.line - 1) as u32,
+      character: range.start.col as u32,
+    },
+    end: Position {
+      line: (range.end.line - 1) as u32,
+      character: range.end.col as u32,
+    },
+  }
+}
+
+pub fn get_lint_references(
+  specifier: &ModuleSpecifier,
+  media_type: &MediaType,
+  source_code: &str,
+) -> Result<Vec<Reference>, AnyError> {
+  let syntax = ast::get_syntax(media_type);
+  let lint_rules = rules::get_recommended_rules();
+  let mut linter = create_linter(syntax, lint_rules);
+  // TODO(@kitsonk) we should consider caching the swc source file versions for
+  // reuse by other processes
+  let (_, lint_diagnostics) =
+    linter.lint(specifier.to_string(), source_code.to_string())?;
+
+  Ok(
+    lint_diagnostics
+      .into_iter()
+      .map(|d| Reference {
+        category: Category::Lint {
+          message: d.message,
+          code: d.code,
+          hint: d.hint,
+        },
+        range: as_lsp_range(&d.range),
+      })
+      .collect(),
+  )
+}
+
+pub fn references_to_diagnostics(
+  references: Vec<Reference>,
+) -> Vec<lsp_types::Diagnostic> {
+  references
+    .into_iter()
+    .map(|r| match r.category {
+      Category::Lint { message, code, .. } => lsp_types::Diagnostic {
+        range: r.range,
+        severity: Some(lsp_types::DiagnosticSeverity::Warning),
+        code: Some(lsp_types::NumberOrString::String(code)),
+        code_description: None,
+        // TODO(@kitsonk) this won't make sense for every diagnostic
+        source: Some("deno-lint".to_string()),
+        message,
+        related_information: None,
+        tags: None, // we should tag unused code
+        data: None,
+      },
+    })
+    .collect()
+}
+
+#[derive(Debug, Default, Clone, PartialEq, Eq)]
+pub struct Dependency {
+  pub is_dynamic: bool,
+  pub maybe_code: Option<ResolvedImport>,
+  pub maybe_type: Option<ResolvedImport>,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum ResolvedImport {
+  Resolved(ModuleSpecifier),
+  Err(String),
+}
+
+pub fn resolve_import(
+  specifier: &str,
+  referrer: &ModuleSpecifier,
+  maybe_import_map: Option<Rc<RefCell<ImportMap>>>,
+) -> ResolvedImport {
+  let maybe_mapped = if let Some(import_map) = maybe_import_map {
+    if let Ok(maybe_specifier) =
+      import_map.borrow().resolve(specifier, referrer.as_str())
+    {
+      maybe_specifier
+    } else {
+      None
+    }
+  } else {
+    None
+  };
+  let remapped = maybe_mapped.is_some();
+  let specifier = if let Some(remapped) = maybe_mapped {
+    remapped
+  } else {
+    match ModuleSpecifier::resolve_import(specifier, referrer.as_str()) {
+      Ok(resolved) => resolved,
+      Err(err) => return ResolvedImport::Err(err.to_string()),
+    }
+  };
+  let referrer_scheme = referrer.as_url().scheme();
+  let specifier_scheme = specifier.as_url().scheme();
+  if referrer_scheme == "https" && specifier_scheme == "http" {
+    return ResolvedImport::Err(
+      "Modules imported via https are not allowed to import http modules."
+        .to_string(),
+    );
+  }
+  if (referrer_scheme == "https" || referrer_scheme == "http")
+    && !(specifier_scheme == "https" || specifier_scheme == "http")
+    && !remapped
+  {
+    return ResolvedImport::Err("Remote modules are not allowed to import local modules.  Consider using a dynamic import instead.".to_string());
+  }
+
+  ResolvedImport::Resolved(specifier)
+}
+
+// TODO(@kitsonk) a lot of this logic is duplicated in module_graph.rs in
+// Module::parse() and should be refactored out to a common function.
+pub fn analyze_dependencies(
+  specifier: &ModuleSpecifier,
+  source: &str,
+  media_type: &MediaType,
+  maybe_import_map: Option<Rc<RefCell<ImportMap>>>,
+) -> Option<(HashMap<String, Dependency>, Option<ResolvedImport>)> {
+  let specifier_str = specifier.to_string();
+  let source_map = Rc::new(swc_common::SourceMap::default());
+  let mut maybe_type = None;
+  if let Ok(parsed_module) =
+    ast::parse_with_source_map(&specifier_str, source, &media_type, source_map)
+  {
+    let mut dependencies = HashMap::<String, Dependency>::new();
+
+    // Parse leading comments for supported triple slash references.
+    for comment in parsed_module.get_leading_comments().iter() {
+      if let Some(ts_reference) = parse_ts_reference(&comment.text) {
+        match ts_reference {
+          TypeScriptReference::Path(import) => {
+            let dep = dependencies.entry(import.clone()).or_default();
+            let resolved_import =
+              resolve_import(&import, specifier, maybe_import_map.clone());
+            dep.maybe_code = Some(resolved_import);
+          }
+          TypeScriptReference::Types(import) => {
+            let resolved_import =
+              resolve_import(&import, specifier, maybe_import_map.clone());
+            if media_type == &MediaType::JavaScript
+              || media_type == &MediaType::JSX
+            {
+              maybe_type = Some(resolved_import)
+            } else {
+              let dep = dependencies.entry(import).or_default();
+              dep.maybe_type = Some(resolved_import);
+            }
+          }
+        }
+      }
+    }
+
+    // Parse ES and type only imports
+    let descriptors = parsed_module.analyze_dependencies();
+    for desc in descriptors.into_iter().filter(|desc| {
+      desc.kind != swc_ecmascript::dep_graph::DependencyKind::Require
+    }) {
+      let resolved_import =
+        resolve_import(&desc.specifier, specifier, maybe_import_map.clone());
+
+      // Check for `@deno-types` pragmas that effect the import
+      let maybe_resolved_type_import =
+        if let Some(comment) = desc.leading_comments.last() {
+          if let Some(deno_types) = parse_deno_types(&comment.text).as_ref() {
+            Some(resolve_import(
+              deno_types,
+              specifier,
+              maybe_import_map.clone(),
+            ))
+          } else {
+            None
+          }
+        } else {
+          None
+        };
+
+      let dep = dependencies.entry(desc.specifier.to_string()).or_default();
+      dep.is_dynamic = desc.is_dynamic;
+      match desc.kind {
+        swc_ecmascript::dep_graph::DependencyKind::ExportType
+        | swc_ecmascript::dep_graph::DependencyKind::ImportType => {
+          dep.maybe_type = Some(resolved_import)
+        }
+        _ => dep.maybe_code = Some(resolved_import),
+      }
+      if maybe_resolved_type_import.is_some() && dep.maybe_type.is_none() {
+        dep.maybe_type = maybe_resolved_type_import;
+      }
+    }
+
+    Some((dependencies, maybe_type))
+  } else {
+    None
+  }
+}
+
+#[cfg(test)]
+mod tests {
+  use super::*;
+
+  #[test]
+  fn test_as_lsp_range() {
+    let fixture = deno_lint::diagnostic::Range {
+      start: deno_lint::diagnostic::Position {
+        line: 1,
+        col: 2,
+        byte_pos: 23,
+      },
+      end: deno_lint::diagnostic::Position {
+        line: 2,
+        col: 0,
+        byte_pos: 33,
+      },
+    };
+    let actual = as_lsp_range(&fixture);
+    assert_eq!(
+      actual,
+      lsp_types::Range {
+        start: lsp_types::Position {
+          line: 0,
+          character: 2,
+        },
+        end: lsp_types::Position {
+          line: 1,
+          character: 0,
+        },
+      }
+    );
+  }
+
+  #[test]
+  fn test_analyze_dependencies() {
+    let specifier =
+      ModuleSpecifier::resolve_url("file:///a.ts").expect("bad specifier");
+    let source = r#"import {
+      Application,
+      Context,
+      Router,
+      Status,
+    } from "https://deno.land/x/oak@v6.3.2/mod.ts";
+    
+    // @deno-types="https://deno.land/x/types/react/index.d.ts";
+    import * as React from "https://cdn.skypack.dev/react";
+    "#;
+    let actual =
+      analyze_dependencies(&specifier, source, &MediaType::TypeScript, None);
+    assert!(actual.is_some());
+    let (actual, maybe_type) = actual.unwrap();
+    assert!(maybe_type.is_none());
+    assert_eq!(actual.len(), 2);
+    assert_eq!(
+      actual.get("https://cdn.skypack.dev/react").cloned(),
+      Some(Dependency {
+        is_dynamic: false,
+        maybe_code: Some(ResolvedImport::Resolved(
+          ModuleSpecifier::resolve_url("https://cdn.skypack.dev/react")
+            .unwrap()
+        )),
+        maybe_type: Some(ResolvedImport::Resolved(
+          ModuleSpecifier::resolve_url(
+            "https://deno.land/x/types/react/index.d.ts"
+          )
+          .unwrap()
+        )),
+      })
+    );
+    assert_eq!(
+      actual.get("https://deno.land/x/oak@v6.3.2/mod.ts").cloned(),
+      Some(Dependency {
+        is_dynamic: false,
+        maybe_code: Some(ResolvedImport::Resolved(
+          ModuleSpecifier::resolve_url("https://deno.land/x/oak@v6.3.2/mod.ts")
+            .unwrap()
+        )),
+        maybe_type: None,
+      })
+    );
+  }
+}
diff --git a/cli/lsp/capabilities.rs b/cli/lsp/capabilities.rs
new file mode 100644
index 000000000..cf8f150ca
--- /dev/null
+++ b/cli/lsp/capabilities.rs
@@ -0,0 +1,59 @@
+// Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
+
+///!
+///! Provides information about what capabilities that are supported by the
+///! language server, which helps determine what messages are sent from the
+///! client.
+///!
+use lsp_types::ClientCapabilities;
+use lsp_types::HoverProviderCapability;
+use lsp_types::OneOf;
+use lsp_types::SaveOptions;
+use lsp_types::ServerCapabilities;
+use lsp_types::TextDocumentSyncCapability;
+use lsp_types::TextDocumentSyncKind;
+use lsp_types::TextDocumentSyncOptions;
+
+pub fn server_capabilities(
+  _client_capabilities: &ClientCapabilities,
+) -> ServerCapabilities {
+  ServerCapabilities {
+    text_document_sync: Some(TextDocumentSyncCapability::Options(
+      TextDocumentSyncOptions {
+        open_close: Some(true),
+        change: Some(TextDocumentSyncKind::Incremental),
+        will_save: None,
+        will_save_wait_until: None,
+        save: Some(SaveOptions::default().into()),
+      },
+    )),
+    hover_provider: Some(HoverProviderCapability::Simple(true)),
+    completion_provider: None,
+    signature_help_provider: None,
+    declaration_provider: None,
+    definition_provider: Some(OneOf::Left(true)),
+    type_definition_provider: None,
+    implementation_provider: None,
+    references_provider: Some(OneOf::Left(true)),
+    document_highlight_provider: Some(OneOf::Left(true)),
+    document_symbol_provider: None,
+    workspace_symbol_provider: None,
+    code_action_provider: None,
+    code_lens_provider: None,
+    document_formatting_provider: Some(OneOf::Left(true)),
+    document_range_formatting_provider: None,
+    document_on_type_formatting_provider: None,
+    selection_range_provider: None,
+    semantic_highlighting: None,
+    folding_range_provider: None,
+    rename_provider: None,
+    document_link_provider: None,
+    color_provider: None,
+    execute_command_provider: None,
+    workspace: None,
+    call_hierarchy_provider: None,
+    semantic_tokens_provider: None,
+    on_type_rename_provider: None,
+    experimental: None,
+  }
+}
diff --git a/cli/lsp/config.rs b/cli/lsp/config.rs
new file mode 100644
index 000000000..ebc145708
--- /dev/null
+++ b/cli/lsp/config.rs
@@ -0,0 +1,49 @@
+// Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
+
+use deno_core::error::AnyError;
+use deno_core::serde::Deserialize;
+use deno_core::serde_json;
+use deno_core::serde_json::Value;
+
+#[derive(Debug, Clone, Default)]
+pub struct ClientCapabilities {
+  pub status_notification: bool,
+}
+
+#[derive(Debug, Clone, Default, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct WorkspaceSettings {
+  pub enable: bool,
+  pub config: Option<String>,
+  pub import_map: Option<String>,
+  pub lint: bool,
+  pub unstable: bool,
+}
+
+#[derive(Debug, Clone, Default)]
+pub struct Config {
+  pub client_capabilities: ClientCapabilities,
+  pub settings: WorkspaceSettings,
+}
+
+impl Config {
+  pub fn update(&mut self, value: Value) -> Result<(), AnyError> {
+    let settings: WorkspaceSettings = serde_json::from_value(value)?;
+    self.settings = settings;
+    Ok(())
+  }
+
+  #[allow(clippy::redundant_closure_call)]
+  pub fn update_capabilities(
+    &mut self,
+    capabilities: &lsp_types::ClientCapabilities,
+  ) {
+    if let Some(experimental) = &capabilities.experimental {
+      let get_bool =
+        |k: &str| experimental.get(k).and_then(|it| it.as_bool()) == Some(true);
+
+      self.client_capabilities.status_notification =
+        get_bool("statusNotification");
+    }
+  }
+}
diff --git a/cli/lsp/diagnostics.rs b/cli/lsp/diagnostics.rs
new file mode 100644
index 000000000..a7f027c1b
--- /dev/null
+++ b/cli/lsp/diagnostics.rs
@@ -0,0 +1,268 @@
+// Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
+
+use super::analysis::get_lint_references;
+use super::analysis::references_to_diagnostics;
+use super::memory_cache::FileId;
+use super::state::ServerStateSnapshot;
+use super::tsc;
+
+use crate::diagnostics;
+use crate::media_type::MediaType;
+
+use deno_core::error::AnyError;
+use deno_core::serde_json;
+use deno_core::serde_json::Value;
+use deno_core::url::Url;
+use deno_core::JsRuntime;
+use std::collections::HashMap;
+use std::collections::HashSet;
+use std::mem;
+
+impl<'a> From<&'a diagnostics::DiagnosticCategory>
+  for lsp_types::DiagnosticSeverity
+{
+  fn from(category: &'a diagnostics::DiagnosticCategory) -> Self {
+    match category {
+      diagnostics::DiagnosticCategory::Error => {
+        lsp_types::DiagnosticSeverity::Error
+      }
+      diagnostics::DiagnosticCategory::Warning => {
+        lsp_types::DiagnosticSeverity::Warning
+      }
+      diagnostics::DiagnosticCategory::Suggestion => {
+        lsp_types::DiagnosticSeverity::Hint
+      }
+      diagnostics::DiagnosticCategory::Message => {
+        lsp_types::DiagnosticSeverity::Information
+      }
+    }
+  }
+}
+
+impl<'a> From<&'a diagnostics::Position> for lsp_types::Position {
+  fn from(pos: &'a diagnostics::Position) -> Self {
+    Self {
+      line: pos.line as u32,
+      character: pos.character as u32,
+    }
+  }
+}
+
+fn to_lsp_range(
+  start: &diagnostics::Position,
+  end: &diagnostics::Position,
+) -> lsp_types::Range {
+  lsp_types::Range {
+    start: start.into(),
+    end: end.into(),
+  }
+}
+
+#[derive(Debug, Clone, Hash, PartialEq, Eq)]
+pub enum DiagnosticSource {
+  Lint,
+  TypeScript,
+}
+
+#[derive(Debug, Default, Clone)]
+pub struct DiagnosticCollection {
+  map: HashMap<(FileId, DiagnosticSource), Vec<lsp_types::Diagnostic>>,
+  versions: HashMap<FileId, i32>,
+  changes: HashSet<FileId>,
+}
+
+impl DiagnosticCollection {
+  pub fn set(
+    &mut self,
+    file_id: FileId,
+    source: DiagnosticSource,
+    version: Option<i32>,
+    diagnostics: Vec<lsp_types::Diagnostic>,
+  ) {
+    self.map.insert((file_id, source), diagnostics);
+    if let Some(version) = version {
+      self.versions.insert(file_id, version);
+    }
+    self.changes.insert(file_id);
+  }
+
+  pub fn diagnostics_for(
+    &self,
+    file_id: FileId,
+    source: DiagnosticSource,
+  ) -> impl Iterator<Item = &lsp_types::Diagnostic> {
+    self.map.get(&(file_id, source)).into_iter().flatten()
+  }
+
+  pub fn get_version(&self, file_id: &FileId) -> Option<i32> {
+    self.versions.get(file_id).cloned()
+  }
+
+  pub fn take_changes(&mut self) -> Option<HashSet<FileId>> {
+    if self.changes.is_empty() {
+      return None;
+    }
+    Some(mem::take(&mut self.changes))
+  }
+}
+
+pub type DiagnosticVec = Vec<(FileId, Option<i32>, Vec<lsp_types::Diagnostic>)>;
+
+pub fn generate_linting_diagnostics(
+  state: &ServerStateSnapshot,
+) -> DiagnosticVec {
+  if !state.config.settings.lint {
+    return Vec::new();
+  }
+  let mut diagnostics = Vec::new();
+  let file_cache = state.file_cache.read().unwrap();
+  for (specifier, doc_data) in state.doc_data.iter() {
+    let file_id = file_cache.lookup(specifier).unwrap();
+    let version = doc_data.version;
+    let current_version = state.diagnostics.get_version(&file_id);
+    if version != current_version {
+      let media_type = MediaType::from(specifier);
+      if let Ok(source_code) = file_cache.get_contents(file_id) {
+        if let Ok(references) =
+          get_lint_references(specifier, &media_type, &source_code)
+        {
+          if !references.is_empty() {
+            diagnostics.push((
+              file_id,
+              version,
+              references_to_diagnostics(references),
+            ));
+          } else {
+            diagnostics.push((file_id, version, Vec::new()));
+          }
+        }
+      } else {
+        error!("Missing file contents for: {}", specifier);
+      }
+    }
+  }
+
+  diagnostics
+}
+
+type TsDiagnostics = Vec<diagnostics::Diagnostic>;
+
+fn get_diagnostic_message(diagnostic: &diagnostics::Diagnostic) -> String {
+  if let Some(message) = diagnostic.message_text.clone() {
+    message
+  } else if let Some(message_chain) = diagnostic.message_chain.clone() {
+    message_chain.format_message(0)
+  } else {
+    "[missing message]".to_string()
+  }
+}
+
+fn to_lsp_related_information(
+  related_information: &Option<Vec<diagnostics::Diagnostic>>,
+) -> Option<Vec<lsp_types::DiagnosticRelatedInformation>> {
+  if let Some(related) = related_information {
+    Some(
+      related
+        .iter()
+        .filter_map(|ri| {
+          if let (Some(source), Some(start), Some(end)) =
+            (&ri.source, &ri.start, &ri.end)
+          {
+            let uri = Url::parse(&source).unwrap();
+            Some(lsp_types::DiagnosticRelatedInformation {
+              location: lsp_types::Location {
+                uri,
+                range: to_lsp_range(start, end),
+              },
+              message: get_diagnostic_message(&ri),
+            })
+          } else {
+            None
+          }
+        })
+        .collect(),
+    )
+  } else {
+    None
+  }
+}
+
+fn ts_json_to_diagnostics(
+  value: Value,
+) -> Result<Vec<lsp_types::Diagnostic>, AnyError> {
+  let ts_diagnostics: TsDiagnostics = serde_json::from_value(value)?;
+  Ok(
+    ts_diagnostics
+      .iter()
+      .filter_map(|d| {
+        if let (Some(start), Some(end)) = (&d.start, &d.end) {
+          Some(lsp_types::Diagnostic {
+            range: to_lsp_range(start, end),
+            severity: Some((&d.category).into()),
+            code: Some(lsp_types::NumberOrString::Number(d.code as i32)),
+            code_description: None,
+            source: Some("deno-ts".to_string()),
+            message: get_diagnostic_message(d),
+            related_information: to_lsp_related_information(
+              &d.related_information,
+            ),
+            tags: match d.code {
+              // These are codes that indicate the variable is unused.
+              6133 | 6192 | 6196 => {
+                Some(vec![lsp_types::DiagnosticTag::Unnecessary])
+              }
+              _ => None,
+            },
+            data: None,
+          })
+        } else {
+          None
+        }
+      })
+      .collect(),
+  )
+}
+
+pub fn generate_ts_diagnostics(
+  state: &ServerStateSnapshot,
+  runtime: &mut JsRuntime,
+) -> Result<DiagnosticVec, AnyError> {
+  if !state.config.settings.enable {
+    return Ok(Vec::new());
+  }
+  let mut diagnostics = Vec::new();
+  let file_cache = state.file_cache.read().unwrap();
+  for (specifier, doc_data) in state.doc_data.iter() {
+    let file_id = file_cache.lookup(specifier).unwrap();
+    let version = doc_data.version;
+    let current_version = state.diagnostics.get_version(&file_id);
+    if version != current_version {
+      // TODO(@kitsonk): consider refactoring to get all diagnostics in one shot
+      // for a file.
+      let request_semantic_diagnostics =
+        tsc::RequestMethod::GetSemanticDiagnostics(specifier.clone());
+      let mut ts_diagnostics = ts_json_to_diagnostics(tsc::request(
+        runtime,
+        state,
+        request_semantic_diagnostics,
+      )?)?;
+      let request_suggestion_diagnostics =
+        tsc::RequestMethod::GetSuggestionDiagnostics(specifier.clone());
+      ts_diagnostics.append(&mut ts_json_to_diagnostics(tsc::request(
+        runtime,
+        state,
+        request_suggestion_diagnostics,
+      )?)?);
+      let request_syntactic_diagnostics =
+        tsc::RequestMethod::GetSyntacticDiagnostics(specifier.clone());
+      ts_diagnostics.append(&mut ts_json_to_diagnostics(tsc::request(
+        runtime,
+        state,
+        request_syntactic_diagnostics,
+      )?)?);
+      diagnostics.push((file_id, version, ts_diagnostics));
+    }
+  }
+
+  Ok(diagnostics)
+}
diff --git a/cli/lsp/dispatch.rs b/cli/lsp/dispatch.rs
new file mode 100644
index 000000000..774bdcef9
--- /dev/null
+++ b/cli/lsp/dispatch.rs
@@ -0,0 +1,185 @@
+// Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
+
+use super::state::ServerState;
+use super::state::ServerStateSnapshot;
+use super::state::Task;
+use super::utils::from_json;
+use super::utils::is_canceled;
+
+use deno_core::error::custom_error;
+use deno_core::error::AnyError;
+use lsp_server::ErrorCode;
+use lsp_server::Notification;
+use lsp_server::Request;
+use lsp_server::RequestId;
+use lsp_server::Response;
+use serde::de::DeserializeOwned;
+use serde::Serialize;
+use std::fmt;
+use std::panic;
+
+pub struct NotificationDispatcher<'a> {
+  pub notification: Option<Notification>,
+  pub server_state: &'a mut ServerState,
+}
+
+impl<'a> NotificationDispatcher<'a> {
+  pub fn on<N>(
+    &mut self,
+    f: fn(&mut ServerState, N::Params) -> Result<(), AnyError>,
+  ) -> Result<&mut Self, AnyError>
+  where
+    N: lsp_types::notification::Notification + 'static,
+    N::Params: DeserializeOwned + Send + 'static,
+  {
+    let notification = match self.notification.take() {
+      Some(it) => it,
+      None => return Ok(self),
+    };
+    let params = match notification.extract::<N::Params>(N::METHOD) {
+      Ok(it) => it,
+      Err(notification) => {
+        self.notification = Some(notification);
+        return Ok(self);
+      }
+    };
+    f(self.server_state, params)?;
+    Ok(self)
+  }
+
+  pub fn finish(&mut self) {
+    if let Some(notification) = &self.notification {
+      if !notification.method.starts_with("$/") {
+        error!("unhandled notification: {:?}", notification);
+      }
+    }
+  }
+}
+
+fn result_to_response<R>(
+  id: RequestId,
+  result: Result<R::Result, AnyError>,
+) -> Response
+where
+  R: lsp_types::request::Request + 'static,
+  R::Params: DeserializeOwned + 'static,
+  R::Result: Serialize + 'static,
+{
+  match result {
+    Ok(response) => Response::new_ok(id, &response),
+    Err(err) => {
+      if is_canceled(&*err) {
+        Response::new_err(
+          id,
+          ErrorCode::ContentModified as i32,
+          "content modified".to_string(),
+        )
+      } else {
+        Response::new_err(id, ErrorCode::InternalError as i32, err.to_string())
+      }
+    }
+  }
+}
+
+pub struct RequestDispatcher<'a> {
+  pub request: Option<Request>,
+  pub server_state: &'a mut ServerState,
+}
+
+impl<'a> RequestDispatcher<'a> {
+  pub fn finish(&mut self) {
+    if let Some(request) = self.request.take() {
+      error!("unknown request: {:?}", request);
+      let response = Response::new_err(
+        request.id,
+        ErrorCode::MethodNotFound as i32,
+        "unknown request".to_string(),
+      );
+      self.server_state.respond(response);
+    }
+  }
+
+  /// Handle a request which will respond to the LSP client asynchronously via
+  /// a spawned thread.
+  pub fn on<R>(
+    &mut self,
+    f: fn(ServerStateSnapshot, R::Params) -> Result<R::Result, AnyError>,
+  ) -> &mut Self
+  where
+    R: lsp_types::request::Request + 'static,
+    R::Params: DeserializeOwned + Send + fmt::Debug + 'static,
+    R::Result: Serialize + 'static,
+  {
+    let (id, params) = match self.parse::<R>() {
+      Some(it) => it,
+      None => return self,
+    };
+    self.server_state.spawn({
+      let state = self.server_state.snapshot();
+      move || {
+        let result = f(state, params);
+        Task::Response(result_to_response::<R>(id, result))
+      }
+    });
+
+    self
+  }
+
+  /// Handle a request which will respond synchronously, returning a result if
+  /// the request cannot be handled or has issues.
+  pub fn on_sync<R>(
+    &mut self,
+    f: fn(&mut ServerState, R::Params) -> Result<R::Result, AnyError>,
+  ) -> Result<&mut Self, AnyError>
+  where
+    R: lsp_types::request::Request + 'static,
+    R::Params: DeserializeOwned + panic::UnwindSafe + fmt::Debug + 'static,
+    R::Result: Serialize + 'static,
+  {
+    let (id, params) = match self.parse::<R>() {
+      Some(it) => it,
+      None => return Ok(self),
+    };
+    let state = panic::AssertUnwindSafe(&mut *self.server_state);
+
+    let response = panic::catch_unwind(move || {
+      let result = f(state.0, params);
+      result_to_response::<R>(id, result)
+    })
+    .map_err(|_err| {
+      custom_error(
+        "SyncTaskPanic",
+        format!("sync task {:?} panicked", R::METHOD),
+      )
+    })?;
+    self.server_state.respond(response);
+    Ok(self)
+  }
+
+  fn parse<R>(&mut self) -> Option<(RequestId, R::Params)>
+  where
+    R: lsp_types::request::Request + 'static,
+    R::Params: DeserializeOwned + 'static,
+  {
+    let request = match &self.request {
+      Some(request) if request.method == R::METHOD => {
+        self.request.take().unwrap()
+      }
+      _ => return None,
+    };
+
+    let response = from_json(R::METHOD, request.params);
+    match response {
+      Ok(params) => Some((request.id, params)),
+      Err(err) => {
+        let response = Response::new_err(
+          request.id,
+          ErrorCode::InvalidParams as i32,
+          err.to_string(),
+        );
+        self.server_state.respond(response);
+        None
+      }
+    }
+  }
+}
diff --git a/cli/lsp/handlers.rs b/cli/lsp/handlers.rs
new file mode 100644
index 000000000..6dd7321c7
--- /dev/null
+++ b/cli/lsp/handlers.rs
@@ -0,0 +1,266 @@
+// Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
+
+use super::lsp_extensions;
+use super::state::ServerState;
+use super::state::ServerStateSnapshot;
+use super::text;
+use super::tsc;
+use super::utils;
+
+use deno_core::error::custom_error;
+use deno_core::error::AnyError;
+use deno_core::serde_json;
+use deno_core::ModuleSpecifier;
+use dprint_plugin_typescript as dprint;
+use lsp_types::DocumentFormattingParams;
+use lsp_types::DocumentHighlight;
+use lsp_types::DocumentHighlightParams;
+use lsp_types::GotoDefinitionParams;
+use lsp_types::GotoDefinitionResponse;
+use lsp_types::Hover;
+use lsp_types::HoverParams;
+use lsp_types::Location;
+use lsp_types::ReferenceParams;
+use lsp_types::TextEdit;
+use std::path::PathBuf;
+
+fn get_line_index(
+  state: &mut ServerState,
+  specifier: &ModuleSpecifier,
+) -> Result<Vec<u32>, AnyError> {
+  let line_index = if specifier.as_url().scheme() == "asset" {
+    if let Some(source) = tsc::get_asset(specifier.as_url().path()) {
+      text::index_lines(source)
+    } else {
+      return Err(custom_error(
+        "NotFound",
+        format!("asset source missing: {}", specifier),
+      ));
+    }
+  } else {
+    let file_cache = state.file_cache.read().unwrap();
+    if let Some(file_id) = file_cache.lookup(specifier) {
+      let file_text = file_cache.get_contents(file_id)?;
+      text::index_lines(&file_text)
+    } else {
+      let mut sources = state.sources.write().unwrap();
+      if let Some(line_index) = sources.get_line_index(specifier) {
+        line_index
+      } else {
+        return Err(custom_error(
+          "NotFound",
+          format!("source for specifier not found: {}", specifier),
+        ));
+      }
+    }
+  };
+  Ok(line_index)
+}
+
+pub fn handle_formatting(
+  state: ServerStateSnapshot,
+  params: DocumentFormattingParams,
+) -> Result<Option<Vec<TextEdit>>, AnyError> {
+  let specifier = utils::normalize_url(params.text_document.uri.clone());
+  let file_cache = state.file_cache.read().unwrap();
+  let file_id = file_cache.lookup(&specifier).unwrap();
+  let file_text = file_cache.get_contents(file_id)?;
+
+  let file_path = if let Ok(file_path) = params.text_document.uri.to_file_path()
+  {
+    file_path
+  } else {
+    PathBuf::from(params.text_document.uri.path())
+  };
+  let config = dprint::configuration::ConfigurationBuilder::new()
+    .deno()
+    .build();
+
+  // TODO(@kitsonk) this could be handled better in `cli/tools/fmt.rs` in the
+  // future.
+  let new_text = dprint::format_text(&file_path, &file_text, &config)
+    .map_err(|e| custom_error("FormatError", e))?;
+
+  let text_edits = text::get_edits(&file_text, &new_text);
+  if text_edits.is_empty() {
+    Ok(None)
+  } else {
+    Ok(Some(text_edits))
+  }
+}
+
+pub fn handle_document_highlight(
+  state: &mut ServerState,
+  params: DocumentHighlightParams,
+) -> Result<Option<Vec<DocumentHighlight>>, AnyError> {
+  let specifier = utils::normalize_url(
+    params.text_document_position_params.text_document.uri,
+  );
+  let line_index = get_line_index(state, &specifier)?;
+  let server_state = state.snapshot();
+  let files_to_search = vec![specifier.clone()];
+  let maybe_document_highlights: Option<Vec<tsc::DocumentHighlights>> =
+    serde_json::from_value(tsc::request(
+      &mut state.ts_runtime,
+      &server_state,
+      tsc::RequestMethod::GetDocumentHighlights((
+        specifier,
+        text::to_char_pos(
+          &line_index,
+          params.text_document_position_params.position,
+        ),
+        files_to_search,
+      )),
+    )?)?;
+
+  if let Some(document_highlights) = maybe_document_highlights {
+    Ok(Some(
+      document_highlights
+        .into_iter()
+        .map(|dh| dh.to_highlight(&line_index))
+        .flatten()
+        .collect(),
+    ))
+  } else {
+    Ok(None)
+  }
+}
+
+pub fn handle_goto_definition(
+  state: &mut ServerState,
+  params: GotoDefinitionParams,
+) -> Result<Option<GotoDefinitionResponse>, AnyError> {
+  let specifier = utils::normalize_url(
+    params.text_document_position_params.text_document.uri,
+  );
+  let line_index = get_line_index(state, &specifier)?;
+  let server_state = state.snapshot();
+  let maybe_definition: Option<tsc::DefinitionInfoAndBoundSpan> =
+    serde_json::from_value(tsc::request(
+      &mut state.ts_runtime,
+      &server_state,
+      tsc::RequestMethod::GetDefinition((
+        specifier,
+        text::to_char_pos(
+          &line_index,
+          params.text_document_position_params.position,
+        ),
+      )),
+    )?)?;
+
+  if let Some(definition) = maybe_definition {
+    Ok(
+      definition
+        .to_definition(&line_index, |s| get_line_index(state, &s).unwrap()),
+    )
+  } else {
+    Ok(None)
+  }
+}
+
+pub fn handle_hover(
+  state: &mut ServerState,
+  params: HoverParams,
+) -> Result<Option<Hover>, AnyError> {
+  let specifier = utils::normalize_url(
+    params.text_document_position_params.text_document.uri,
+  );
+  let line_index = get_line_index(state, &specifier)?;
+  let server_state = state.snapshot();
+  let maybe_quick_info: Option<tsc::QuickInfo> =
+    serde_json::from_value(tsc::request(
+      &mut state.ts_runtime,
+      &server_state,
+      tsc::RequestMethod::GetQuickInfo((
+        specifier,
+        text::to_char_pos(
+          &line_index,
+          params.text_document_position_params.position,
+        ),
+      )),
+    )?)?;
+
+  if let Some(quick_info) = maybe_quick_info {
+    Ok(Some(quick_info.to_hover(&line_index)))
+  } else {
+    Ok(None)
+  }
+}
+
+pub fn handle_references(
+  state: &mut ServerState,
+  params: ReferenceParams,
+) -> Result<Option<Vec<Location>>, AnyError> {
+  let specifier =
+    utils::normalize_url(params.text_document_position.text_document.uri);
+  let line_index = get_line_index(state, &specifier)?;
+  let server_state = state.snapshot();
+  let maybe_references: Option<Vec<tsc::ReferenceEntry>> =
+    serde_json::from_value(tsc::request(
+      &mut state.ts_runtime,
+      &server_state,
+      tsc::RequestMethod::GetReferences((
+        specifier,
+        text::to_char_pos(&line_index, params.text_document_position.position),
+      )),
+    )?)?;
+
+  if let Some(references) = maybe_references {
+    let mut results = Vec::new();
+    for reference in references {
+      if !params.context.include_declaration && reference.is_definition {
+        continue;
+      }
+      let reference_specifier =
+        ModuleSpecifier::resolve_url(&reference.file_name).unwrap();
+      let line_index = get_line_index(state, &reference_specifier)?;
+      results.push(reference.to_location(&line_index));
+    }
+
+    Ok(Some(results))
+  } else {
+    Ok(None)
+  }
+}
+
+pub fn handle_virtual_text_document(
+  state: ServerStateSnapshot,
+  params: lsp_extensions::VirtualTextDocumentParams,
+) -> Result<String, AnyError> {
+  let specifier = utils::normalize_url(params.text_document.uri);
+  let url = specifier.as_url();
+  let contents = if url.as_str() == "deno:///status.md" {
+    let file_cache = state.file_cache.read().unwrap();
+    format!(
+      r#"# Deno Language Server Status
+
+- Documents in memory: {}
+
+"#,
+      file_cache.len()
+    )
+  } else {
+    match url.scheme() {
+      "asset" => {
+        if let Some(text) = tsc::get_asset(url.path()) {
+          text.to_string()
+        } else {
+          error!("Missing asset: {}", specifier);
+          "".to_string()
+        }
+      }
+      _ => {
+        let mut sources = state.sources.write().unwrap();
+        if let Some(text) = sources.get_text(&specifier) {
+          text
+        } else {
+          return Err(custom_error(
+            "NotFound",
+            format!("The cached sources was not found: {}", specifier),
+          ));
+        }
+      }
+    }
+  };
+  Ok(contents)
+}
diff --git a/cli/lsp/lsp_extensions.rs b/cli/lsp/lsp_extensions.rs
new file mode 100644
index 000000000..eb0a62464
--- /dev/null
+++ b/cli/lsp/lsp_extensions.rs
@@ -0,0 +1,26 @@
+// Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
+
+///!
+///! Extensions to the language service protocol that are specific to Deno.
+///!
+use deno_core::serde::Deserialize;
+use deno_core::serde::Serialize;
+use lsp_types::request::Request;
+use lsp_types::TextDocumentIdentifier;
+
+#[derive(Debug, Deserialize, Serialize)]
+#[serde(rename_all = "camelCase")]
+pub struct VirtualTextDocumentParams {
+  pub text_document: TextDocumentIdentifier,
+}
+
+/// Request a _virtual_ text document from the server. Used for example to
+/// provide a status document of the language server which can be viewed in the
+/// IDE.
+pub enum VirtualTextDocument {}
+
+impl Request for VirtualTextDocument {
+  type Params = VirtualTextDocumentParams;
+  type Result = String;
+  const METHOD: &'static str = "deno/virtualTextDocument";
+}
diff --git a/cli/lsp/memory_cache.rs b/cli/lsp/memory_cache.rs
new file mode 100644
index 000000000..75c5bdb25
--- /dev/null
+++ b/cli/lsp/memory_cache.rs
@@ -0,0 +1,126 @@
+// Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
+
+use deno_core::error::AnyError;
+use deno_core::ModuleSpecifier;
+use std::collections::HashMap;
+use std::fmt;
+use std::mem;
+
+#[derive(Copy, Clone, Debug, Ord, PartialOrd, Eq, PartialEq, Hash)]
+pub struct FileId(pub u32);
+
+#[derive(Eq, PartialEq, Copy, Clone, Debug)]
+pub enum ChangeKind {
+  Create,
+  Modify,
+  Delete,
+}
+
+pub struct ChangedFile {
+  pub change_kind: ChangeKind,
+  pub file_id: FileId,
+}
+
+#[derive(Default)]
+struct SpecifierInterner {
+  map: HashMap<ModuleSpecifier, FileId>,
+  vec: Vec<ModuleSpecifier>,
+}
+
+impl SpecifierInterner {
+  pub fn get(&self, specifier: &ModuleSpecifier) -> Option<FileId> {
+    self.map.get(specifier).copied()
+  }
+
+  pub fn intern(&mut self, specifier: ModuleSpecifier) -> FileId {
+    if let Some(id) = self.get(&specifier) {
+      return id;
+    }
+    let id = FileId(self.vec.len() as u32);
+    self.map.insert(specifier.clone(), id);
+    self.vec.push(specifier);
+    id
+  }
+
+  pub fn lookup(&self, id: FileId) -> &ModuleSpecifier {
+    &self.vec[id.0 as usize]
+  }
+}
+
+#[derive(Default)]
+pub struct MemoryCache {
+  data: Vec<Option<Vec<u8>>>,
+  interner: SpecifierInterner,
+  changes: Vec<ChangedFile>,
+}
+
+impl MemoryCache {
+  fn alloc_file_id(&mut self, specifier: ModuleSpecifier) -> FileId {
+    let file_id = self.interner.intern(specifier);
+    let idx = file_id.0 as usize;
+    let len = self.data.len().max(idx + 1);
+    self.data.resize_with(len, || None);
+    file_id
+  }
+
+  fn get(&self, file_id: FileId) -> &Option<Vec<u8>> {
+    &self.data[file_id.0 as usize]
+  }
+
+  pub fn get_contents(&self, file_id: FileId) -> Result<String, AnyError> {
+    String::from_utf8(self.get(file_id).as_deref().unwrap().to_vec())
+      .map_err(|err| err.into())
+  }
+
+  fn get_mut(&mut self, file_id: FileId) -> &mut Option<Vec<u8>> {
+    &mut self.data[file_id.0 as usize]
+  }
+
+  pub fn get_specifier(&self, file_id: FileId) -> &ModuleSpecifier {
+    self.interner.lookup(file_id)
+  }
+
+  pub fn len(&self) -> usize {
+    self.data.len()
+  }
+
+  pub fn lookup(&self, specifier: &ModuleSpecifier) -> Option<FileId> {
+    self
+      .interner
+      .get(specifier)
+      .filter(|&it| self.get(it).is_some())
+  }
+
+  pub fn set_contents(
+    &mut self,
+    specifier: ModuleSpecifier,
+    contents: Option<Vec<u8>>,
+  ) {
+    let file_id = self.alloc_file_id(specifier);
+    let change_kind = match (self.get(file_id), &contents) {
+      (None, None) => return,
+      (None, Some(_)) => ChangeKind::Create,
+      (Some(_), None) => ChangeKind::Delete,
+      (Some(old), Some(new)) if old == new => return,
+      (Some(_), Some(_)) => ChangeKind::Modify,
+    };
+
+    *self.get_mut(file_id) = contents;
+    self.changes.push(ChangedFile {
+      file_id,
+      change_kind,
+    })
+  }
+
+  pub fn take_changes(&mut self) -> Vec<ChangedFile> {
+    mem::take(&mut self.changes)
+  }
+}
+
+impl fmt::Debug for MemoryCache {
+  fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    f.debug_struct("MemoryCache")
+      .field("no_files", &self.data.len())
+      .finish()
+  }
+}
diff --git a/cli/lsp/mod.rs b/cli/lsp/mod.rs
new file mode 100644
index 000000000..c26c5d89e
--- /dev/null
+++ b/cli/lsp/mod.rs
@@ -0,0 +1,415 @@
+// Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
+
+mod analysis;
+mod capabilities;
+mod config;
+mod diagnostics;
+mod dispatch;
+mod handlers;
+mod lsp_extensions;
+mod memory_cache;
+mod sources;
+mod state;
+mod text;
+mod tsc;
+mod utils;
+
+use config::Config;
+use diagnostics::DiagnosticSource;
+use dispatch::NotificationDispatcher;
+use dispatch::RequestDispatcher;
+use state::DocumentData;
+use state::Event;
+use state::ServerState;
+use state::Status;
+use state::Task;
+use text::apply_content_changes;
+
+use crate::tsc_config::TsConfig;
+
+use crossbeam_channel::Receiver;
+use deno_core::error::custom_error;
+use deno_core::error::AnyError;
+use deno_core::serde_json;
+use deno_core::serde_json::json;
+use lsp_server::Connection;
+use lsp_server::ErrorCode;
+use lsp_server::Message;
+use lsp_server::Notification;
+use lsp_server::Request;
+use lsp_server::RequestId;
+use lsp_server::Response;
+use lsp_types::notification::Notification as _;
+use lsp_types::Diagnostic;
+use lsp_types::InitializeParams;
+use lsp_types::InitializeResult;
+use lsp_types::ServerInfo;
+use std::env;
+use std::time::Instant;
+
+pub fn start() -> Result<(), AnyError> {
+  info!("Starting Deno language server...");
+
+  let (connection, io_threads) = Connection::stdio();
+  let (initialize_id, initialize_params) = connection.initialize_start()?;
+  let initialize_params: InitializeParams =
+    serde_json::from_value(initialize_params)?;
+
+  let capabilities =
+    capabilities::server_capabilities(&initialize_params.capabilities);
+
+  let version = format!(
+    "{} ({}, {})",
+    crate::version::deno(),
+    env!("PROFILE"),
+    env!("TARGET")
+  );
+
+  info!("  version: {}", version);
+
+  let initialize_result = InitializeResult {
+    capabilities,
+    server_info: Some(ServerInfo {
+      name: "deno-language-server".to_string(),
+      version: Some(version),
+    }),
+  };
+  let initialize_result = serde_json::to_value(initialize_result)?;
+
+  connection.initialize_finish(initialize_id, initialize_result)?;
+
+  if let Some(client_info) = initialize_params.client_info {
+    info!(
+      "Connected to \"{}\" {}",
+      client_info.name,
+      client_info.version.unwrap_or_default()
+    );
+  }
+
+  let mut config = Config::default();
+  if let Some(value) = initialize_params.initialization_options {
+    config.update(value)?;
+  }
+  config.update_capabilities(&initialize_params.capabilities);
+
+  let mut server_state = state::ServerState::new(connection.sender, config);
+  let state = server_state.snapshot();
+
+  // TODO(@kitsonk) need to make this configurable, respect unstable
+  let ts_config = TsConfig::new(json!({
+    "allowJs": true,
+    "experimentalDecorators": true,
+    "isolatedModules": true,
+    "lib": ["deno.ns", "deno.window"],
+    "module": "esnext",
+    "noEmit": true,
+    "strict": true,
+    "target": "esnext",
+  }));
+  tsc::request(
+    &mut server_state.ts_runtime,
+    &state,
+    tsc::RequestMethod::Configure(ts_config),
+  )?;
+
+  // listen for events and run the main loop
+  server_state.run(connection.receiver)?;
+
+  io_threads.join()?;
+  info!("Stop language server");
+  Ok(())
+}
+
+impl ServerState {
+  fn handle_event(&mut self, event: Event) -> Result<(), AnyError> {
+    let received = Instant::now();
+    debug!("handle_event({:?})", event);
+
+    match event {
+      Event::Message(message) => match message {
+        Message::Request(request) => self.on_request(request, received)?,
+        Message::Notification(notification) => {
+          self.on_notification(notification)?
+        }
+        Message::Response(response) => self.complete_request(response),
+      },
+      Event::Task(mut task) => loop {
+        match task {
+          Task::Response(response) => self.respond(response),
+          Task::Diagnostics((source, diagnostics_per_file)) => {
+            for (file_id, version, diagnostics) in diagnostics_per_file {
+              self.diagnostics.set(
+                file_id,
+                source.clone(),
+                version,
+                diagnostics,
+              );
+            }
+          }
+        }
+
+        task = match self.task_receiver.try_recv() {
+          Ok(task) => task,
+          Err(_) => break,
+        };
+      },
+    }
+
+    // process server sent notifications, like diagnostics
+    // TODO(@kitsonk) currently all of these refresh all open documents, though
+    // in a lot of cases, like linting, we would only care about the files
+    // themselves that have changed
+    if self.process_changes() {
+      debug!("process changes");
+      let state = self.snapshot();
+      self.spawn(move || {
+        let diagnostics = diagnostics::generate_linting_diagnostics(&state);
+        Task::Diagnostics((DiagnosticSource::Lint, diagnostics))
+      });
+      // TODO(@kitsonk) isolates do not have Send to be safely sent between
+      // threads, so I am not sure this is the best way to handle queuing up of
+      // getting the diagnostics from the isolate.
+      let state = self.snapshot();
+      let diagnostics =
+        diagnostics::generate_ts_diagnostics(&state, &mut self.ts_runtime)?;
+      self.spawn(move || {
+        Task::Diagnostics((DiagnosticSource::TypeScript, diagnostics))
+      });
+    }
+
+    // process any changes to the diagnostics
+    if let Some(diagnostic_changes) = self.diagnostics.take_changes() {
+      debug!("diagnostics have changed");
+      let state = self.snapshot();
+      for file_id in diagnostic_changes {
+        let file_cache = state.file_cache.read().unwrap();
+        // TODO(@kitsonk) not totally happy with the way we collect and store
+        // different types of diagnostics and offer them up to the client, we
+        // do need to send "empty" vectors though when a particular feature is
+        // disabled, otherwise the client will not clear down previous
+        // diagnostics
+        let mut diagnostics: Vec<Diagnostic> = if state.config.settings.lint {
+          self
+            .diagnostics
+            .diagnostics_for(file_id, DiagnosticSource::Lint)
+            .cloned()
+            .collect()
+        } else {
+          vec![]
+        };
+        if state.config.settings.enable {
+          diagnostics.extend(
+            self
+              .diagnostics
+              .diagnostics_for(file_id, DiagnosticSource::TypeScript)
+              .cloned(),
+          );
+        }
+        let specifier = file_cache.get_specifier(file_id);
+        let uri = specifier.as_url().clone();
+        let version = if let Some(doc_data) = self.doc_data.get(specifier) {
+          doc_data.version
+        } else {
+          None
+        };
+        self.send_notification::<lsp_types::notification::PublishDiagnostics>(
+          lsp_types::PublishDiagnosticsParams {
+            uri,
+            diagnostics,
+            version,
+          },
+        );
+      }
+    }
+
+    Ok(())
+  }
+
+  fn on_notification(
+    &mut self,
+    notification: Notification,
+  ) -> Result<(), AnyError> {
+    NotificationDispatcher {
+      notification: Some(notification),
+      server_state: self,
+    }
+    // TODO(@kitsonk) this is just stubbed out and we don't currently actually
+    // cancel in progress work, though most of our work isn't long running
+    .on::<lsp_types::notification::Cancel>(|state, params| {
+      let id: RequestId = match params.id {
+        lsp_types::NumberOrString::Number(id) => id.into(),
+        lsp_types::NumberOrString::String(id) => id.into(),
+      };
+      state.cancel(id);
+      Ok(())
+    })?
+    .on::<lsp_types::notification::DidOpenTextDocument>(|state, params| {
+      if params.text_document.uri.scheme() == "deno" {
+        // we can ignore virtual text documents opening, as they don't need to
+        // be tracked in memory, as they are static assets that won't change
+        // already managed by the language service
+        return Ok(());
+      }
+      let specifier = utils::normalize_url(params.text_document.uri);
+      if state
+        .doc_data
+        .insert(
+          specifier.clone(),
+          DocumentData::new(
+            specifier.clone(),
+            params.text_document.version,
+            &params.text_document.text,
+            None,
+          ),
+        )
+        .is_some()
+      {
+        error!("duplicate DidOpenTextDocument: {}", specifier);
+      }
+      state
+        .file_cache
+        .write()
+        .unwrap()
+        .set_contents(specifier, Some(params.text_document.text.into_bytes()));
+
+      Ok(())
+    })?
+    .on::<lsp_types::notification::DidChangeTextDocument>(|state, params| {
+      let specifier = utils::normalize_url(params.text_document.uri);
+      let mut file_cache = state.file_cache.write().unwrap();
+      let file_id = file_cache.lookup(&specifier).unwrap();
+      let mut content = file_cache.get_contents(file_id)?;
+      apply_content_changes(&mut content, params.content_changes);
+      let doc_data = state.doc_data.get_mut(&specifier).unwrap();
+      doc_data.update(params.text_document.version, &content, None);
+      file_cache.set_contents(specifier, Some(content.into_bytes()));
+
+      Ok(())
+    })?
+    .on::<lsp_types::notification::DidCloseTextDocument>(|state, params| {
+      if params.text_document.uri.scheme() == "deno" {
+        // we can ignore virtual text documents opening, as they don't need to
+        // be tracked in memory, as they are static assets that won't change
+        // already managed by the language service
+        return Ok(());
+      }
+      let specifier = utils::normalize_url(params.text_document.uri);
+      if state.doc_data.remove(&specifier).is_none() {
+        error!("orphaned document: {}", specifier);
+      }
+      // TODO(@kitsonk) should we do garbage collection on the diagnostics?
+
+      Ok(())
+    })?
+    .on::<lsp_types::notification::DidSaveTextDocument>(|_state, _params| {
+      // nothing to do yet... cleanup things?
+
+      Ok(())
+    })?
+    .on::<lsp_types::notification::DidChangeConfiguration>(|state, _params| {
+      state.send_request::<lsp_types::request::WorkspaceConfiguration>(
+        lsp_types::ConfigurationParams {
+          items: vec![lsp_types::ConfigurationItem {
+            scope_uri: None,
+            section: Some("deno".to_string()),
+          }],
+        },
+        |state, response| {
+          let Response { error, result, .. } = response;
+
+          match (error, result) {
+            (Some(err), _) => {
+              error!("failed to fetch the extension settings: {:?}", err);
+            }
+            (None, Some(config)) => {
+              if let Some(config) = config.get(0) {
+                if let Err(err) = state.config.update(config.clone()) {
+                  error!("failed to update settings: {}", err);
+                }
+              }
+            }
+            (None, None) => {
+              error!("received empty extension settings from the client");
+            }
+          }
+        },
+      );
+
+      Ok(())
+    })?
+    .finish();
+
+    Ok(())
+  }
+
+  fn on_request(
+    &mut self,
+    request: Request,
+    received: Instant,
+  ) -> Result<(), AnyError> {
+    self.register_request(&request, received);
+
+    if self.shutdown_requested {
+      self.respond(Response::new_err(
+        request.id,
+        ErrorCode::InvalidRequest as i32,
+        "Shutdown already requested".to_string(),
+      ));
+      return Ok(());
+    }
+
+    if self.status == Status::Loading && request.method != "shutdown" {
+      self.respond(Response::new_err(
+        request.id,
+        ErrorCode::ContentModified as i32,
+        "Deno Language Server is still loading...".to_string(),
+      ));
+      return Ok(());
+    }
+
+    RequestDispatcher {
+      request: Some(request),
+      server_state: self,
+    }
+    .on_sync::<lsp_types::request::Shutdown>(|s, ()| {
+      s.shutdown_requested = true;
+      Ok(())
+    })?
+    .on_sync::<lsp_types::request::DocumentHighlightRequest>(
+      handlers::handle_document_highlight,
+    )?
+    .on_sync::<lsp_types::request::GotoDefinition>(
+      handlers::handle_goto_definition,
+    )?
+    .on_sync::<lsp_types::request::HoverRequest>(handlers::handle_hover)?
+    .on_sync::<lsp_types::request::References>(handlers::handle_references)?
+    .on::<lsp_types::request::Formatting>(handlers::handle_formatting)
+    .on::<lsp_extensions::VirtualTextDocument>(
+      handlers::handle_virtual_text_document,
+    )
+    .finish();
+
+    Ok(())
+  }
+
+  /// Start consuming events from the provided receiver channel.
+  pub fn run(mut self, inbox: Receiver<Message>) -> Result<(), AnyError> {
+    // currently we don't need to do any other loading or tasks, so as soon as
+    // we run we are "ready"
+    self.transition(Status::Ready);
+
+    while let Some(event) = self.next_event(&inbox) {
+      if let Event::Message(Message::Notification(notification)) = &event {
+        if notification.method == lsp_types::notification::Exit::METHOD {
+          return Ok(());
+        }
+      }
+      self.handle_event(event)?
+    }
+
+    Err(custom_error(
+      "ClientError",
+      "Client exited without proper shutdown sequence.",
+    ))
+  }
+}
diff --git a/cli/lsp/sources.rs b/cli/lsp/sources.rs
new file mode 100644
index 000000000..4f80044a2
--- /dev/null
+++ b/cli/lsp/sources.rs
@@ -0,0 +1,372 @@
+// Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
+
+use super::analysis;
+use super::text;
+
+use crate::file_fetcher::get_source_from_bytes;
+use crate::file_fetcher::map_content_type;
+use crate::http_cache;
+use crate::http_cache::HttpCache;
+use crate::media_type::MediaType;
+use crate::text_encoding;
+
+use deno_core::serde_json;
+use deno_core::ModuleSpecifier;
+use std::collections::HashMap;
+use std::fs;
+use std::path::Path;
+use std::path::PathBuf;
+use std::time::SystemTime;
+
+#[derive(Debug, Clone, Default)]
+struct Metadata {
+  dependencies: Option<HashMap<String, analysis::Dependency>>,
+  maybe_types: Option<analysis::ResolvedImport>,
+  media_type: MediaType,
+  source: String,
+  version: String,
+}
+
+#[derive(Debug, Clone, Default)]
+pub struct Sources {
+  http_cache: HttpCache,
+  metadata: HashMap<ModuleSpecifier, Metadata>,
+  redirects: HashMap<ModuleSpecifier, ModuleSpecifier>,
+  remotes: HashMap<ModuleSpecifier, PathBuf>,
+}
+
+impl Sources {
+  pub fn new(location: &Path) -> Self {
+    Self {
+      http_cache: HttpCache::new(location),
+      ..Default::default()
+    }
+  }
+
+  pub fn contains(&mut self, specifier: &ModuleSpecifier) -> bool {
+    if let Some(specifier) = self.resolve_specifier(specifier) {
+      if self.get_metadata(&specifier).is_some() {
+        return true;
+      }
+    }
+    false
+  }
+
+  pub fn get_length(&mut self, specifier: &ModuleSpecifier) -> Option<usize> {
+    let specifier = self.resolve_specifier(specifier)?;
+    let metadata = self.get_metadata(&specifier)?;
+    Some(metadata.source.chars().count())
+  }
+
+  pub fn get_line_index(
+    &mut self,
+    specifier: &ModuleSpecifier,
+  ) -> Option<Vec<u32>> {
+    let specifier = self.resolve_specifier(specifier)?;
+    let metadata = self.get_metadata(&specifier)?;
+    Some(text::index_lines(&metadata.source))
+  }
+
+  pub fn get_media_type(
+    &mut self,
+    specifier: &ModuleSpecifier,
+  ) -> Option<MediaType> {
+    let specifier = self.resolve_specifier(specifier)?;
+    let metadata = self.get_metadata(&specifier)?;
+    Some(metadata.media_type)
+  }
+
+  fn get_metadata(&mut self, specifier: &ModuleSpecifier) -> Option<Metadata> {
+    if let Some(metadata) = self.metadata.get(specifier).cloned() {
+      if let Some(current_version) = self.get_script_version(specifier) {
+        if metadata.version == current_version {
+          return Some(metadata);
+        }
+      }
+    }
+    let version = self.get_script_version(specifier)?;
+    let path = self.get_path(specifier)?;
+    if let Ok(bytes) = fs::read(path) {
+      if specifier.as_url().scheme() == "file" {
+        let charset = text_encoding::detect_charset(&bytes).to_string();
+        if let Ok(source) = get_source_from_bytes(bytes, Some(charset)) {
+          let media_type = MediaType::from(specifier);
+          let mut maybe_types = None;
+          let dependencies = if let Some((dependencies, mt)) =
+            analysis::analyze_dependencies(
+              &specifier,
+              &source,
+              &media_type,
+              None,
+            ) {
+            maybe_types = mt;
+            Some(dependencies)
+          } else {
+            None
+          };
+          let metadata = Metadata {
+            dependencies,
+            maybe_types,
+            media_type,
+            source,
+            version,
+          };
+          self.metadata.insert(specifier.clone(), metadata.clone());
+          Some(metadata)
+        } else {
+          None
+        }
+      } else {
+        let headers = self.get_remote_headers(specifier)?;
+        let maybe_content_type = headers.get("content-type").cloned();
+        let (media_type, maybe_charset) =
+          map_content_type(specifier, maybe_content_type);
+        if let Ok(source) = get_source_from_bytes(bytes, maybe_charset) {
+          let mut maybe_types =
+            if let Some(types) = headers.get("x-typescript-types") {
+              Some(analysis::resolve_import(types, &specifier, None))
+            } else {
+              None
+            };
+          let dependencies = if let Some((dependencies, mt)) =
+            analysis::analyze_dependencies(
+              &specifier,
+              &source,
+              &media_type,
+              None,
+            ) {
+            if maybe_types.is_none() {
+              maybe_types = mt;
+            }
+            Some(dependencies)
+          } else {
+            None
+          };
+          let metadata = Metadata {
+            dependencies,
+            maybe_types,
+            media_type,
+            source,
+            version,
+          };
+          self.metadata.insert(specifier.clone(), metadata.clone());
+          Some(metadata)
+        } else {
+          None
+        }
+      }
+    } else {
+      None
+    }
+  }
+
+  fn get_path(&mut self, specifier: &ModuleSpecifier) -> Option<PathBuf> {
+    let specifier = self.resolve_specifier(specifier)?;
+    if specifier.as_url().scheme() == "file" {
+      if let Ok(path) = specifier.as_url().to_file_path() {
+        Some(path)
+      } else {
+        None
+      }
+    } else if let Some(path) = self.remotes.get(&specifier) {
+      Some(path.clone())
+    } else {
+      let path = self.http_cache.get_cache_filename(&specifier.as_url());
+      if path.is_file() {
+        self.remotes.insert(specifier.clone(), path.clone());
+        Some(path)
+      } else {
+        None
+      }
+    }
+  }
+
+  fn get_remote_headers(
+    &self,
+    specifier: &ModuleSpecifier,
+  ) -> Option<HashMap<String, String>> {
+    let cache_filename = self.http_cache.get_cache_filename(specifier.as_url());
+    let metadata_path = http_cache::Metadata::filename(&cache_filename);
+    if let Ok(metadata) = fs::read_to_string(metadata_path) {
+      if let Ok(metadata) =
+        serde_json::from_str::<'_, http_cache::Metadata>(&metadata)
+      {
+        return Some(metadata.headers);
+      }
+    }
+    None
+  }
+
+  pub fn get_script_version(
+    &mut self,
+    specifier: &ModuleSpecifier,
+  ) -> Option<String> {
+    if let Some(path) = self.get_path(specifier) {
+      if let Ok(metadata) = fs::metadata(path) {
+        if let Ok(modified) = metadata.modified() {
+          return if let Ok(n) = modified.duration_since(SystemTime::UNIX_EPOCH)
+          {
+            Some(format!("{}", n.as_millis()))
+          } else {
+            Some("1".to_string())
+          };
+        } else {
+          return Some("1".to_string());
+        }
+      }
+    }
+    None
+  }
+
+  pub fn get_text(&mut self, specifier: &ModuleSpecifier) -> Option<String> {
+    let specifier = self.resolve_specifier(specifier)?;
+    let metadata = self.get_metadata(&specifier)?;
+    Some(metadata.source)
+  }
+
+  fn resolution_result(
+    &mut self,
+    resolved_specifier: &ModuleSpecifier,
+  ) -> Option<(ModuleSpecifier, MediaType)> {
+    let resolved_specifier = self.resolve_specifier(resolved_specifier)?;
+    let media_type =
+      if let Some(metadata) = self.metadata.get(&resolved_specifier) {
+        metadata.media_type
+      } else {
+        MediaType::from(&resolved_specifier)
+      };
+    Some((resolved_specifier, media_type))
+  }
+
+  pub fn resolve_import(
+    &mut self,
+    specifier: &str,
+    referrer: &ModuleSpecifier,
+  ) -> Option<(ModuleSpecifier, MediaType)> {
+    let referrer = self.resolve_specifier(referrer)?;
+    let metadata = self.get_metadata(&referrer)?;
+    let dependencies = &metadata.dependencies?;
+    let dependency = dependencies.get(specifier)?;
+    if let Some(type_dependency) = &dependency.maybe_type {
+      if let analysis::ResolvedImport::Resolved(resolved_specifier) =
+        type_dependency
+      {
+        self.resolution_result(resolved_specifier)
+      } else {
+        None
+      }
+    } else {
+      let code_dependency = &dependency.maybe_code.clone()?;
+      if let analysis::ResolvedImport::Resolved(resolved_specifier) =
+        code_dependency
+      {
+        self.resolution_result(resolved_specifier)
+      } else {
+        None
+      }
+    }
+  }
+
+  fn resolve_specifier(
+    &mut self,
+    specifier: &ModuleSpecifier,
+  ) -> Option<ModuleSpecifier> {
+    if specifier.as_url().scheme() == "file" {
+      if let Ok(path) = specifier.as_url().to_file_path() {
+        if path.is_file() {
+          return Some(specifier.clone());
+        }
+      }
+    } else {
+      if let Some(specifier) = self.redirects.get(specifier) {
+        return Some(specifier.clone());
+      }
+      if let Some(redirect) = self.resolve_remote_specifier(specifier, 10) {
+        self.redirects.insert(specifier.clone(), redirect.clone());
+        return Some(redirect);
+      }
+    }
+    None
+  }
+
+  fn resolve_remote_specifier(
+    &self,
+    specifier: &ModuleSpecifier,
+    redirect_limit: isize,
+  ) -> Option<ModuleSpecifier> {
+    let cached_filename =
+      self.http_cache.get_cache_filename(specifier.as_url());
+    if redirect_limit >= 0 && cached_filename.is_file() {
+      if let Some(headers) = self.get_remote_headers(specifier) {
+        if let Some(redirect_to) = headers.get("location") {
+          if let Ok(redirect) =
+            ModuleSpecifier::resolve_import(redirect_to, specifier.as_str())
+          {
+            return self
+              .resolve_remote_specifier(&redirect, redirect_limit - 1);
+          }
+        } else {
+          return Some(specifier.clone());
+        }
+      }
+    }
+    None
+  }
+}
+
+#[cfg(test)]
+mod tests {
+  use super::*;
+  use std::env;
+  use tempfile::TempDir;
+
+  fn setup() -> (Sources, PathBuf) {
+    let temp_dir = TempDir::new().expect("could not create temp dir");
+    let location = temp_dir.path().join("deps");
+    let sources = Sources::new(&location);
+    (sources, location)
+  }
+
+  #[test]
+  fn test_sources_get_script_version() {
+    let (mut sources, _) = setup();
+    let c = PathBuf::from(env::var_os("CARGO_MANIFEST_DIR").unwrap());
+    let tests = c.join("tests");
+    let specifier = ModuleSpecifier::resolve_path(
+      &tests.join("001_hello.js").to_string_lossy(),
+    )
+    .unwrap();
+    let actual = sources.get_script_version(&specifier);
+    assert!(actual.is_some());
+  }
+
+  #[test]
+  fn test_sources_get_text() {
+    let (mut sources, _) = setup();
+    let c = PathBuf::from(env::var_os("CARGO_MANIFEST_DIR").unwrap());
+    let tests = c.join("tests");
+    let specifier = ModuleSpecifier::resolve_path(
+      &tests.join("001_hello.js").to_string_lossy(),
+    )
+    .unwrap();
+    let actual = sources.get_text(&specifier);
+    assert!(actual.is_some());
+    let actual = actual.unwrap();
+    assert_eq!(actual, "console.log(\"Hello World\");\n");
+  }
+
+  #[test]
+  fn test_sources_get_length() {
+    let (mut sources, _) = setup();
+    let c = PathBuf::from(env::var_os("CARGO_MANIFEST_DIR").unwrap());
+    let tests = c.join("tests");
+    let specifier = ModuleSpecifier::resolve_path(
+      &tests.join("001_hello.js").to_string_lossy(),
+    )
+    .unwrap();
+    let actual = sources.get_length(&specifier);
+    assert!(actual.is_some());
+    let actual = actual.unwrap();
+    assert_eq!(actual, 28);
+  }
+}
diff --git a/cli/lsp/state.rs b/cli/lsp/state.rs
new file mode 100644
index 000000000..18a1e4023
--- /dev/null
+++ b/cli/lsp/state.rs
@@ -0,0 +1,292 @@
+// Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
+
+use super::analysis;
+use super::config::Config;
+use super::diagnostics::DiagnosticCollection;
+use super::diagnostics::DiagnosticSource;
+use super::diagnostics::DiagnosticVec;
+use super::memory_cache::MemoryCache;
+use super::sources::Sources;
+use super::tsc;
+use super::utils::notification_is;
+
+use crate::deno_dir;
+use crate::import_map::ImportMap;
+use crate::media_type::MediaType;
+
+use crossbeam_channel::select;
+use crossbeam_channel::unbounded;
+use crossbeam_channel::Receiver;
+use crossbeam_channel::Sender;
+use deno_core::JsRuntime;
+use deno_core::ModuleSpecifier;
+use lsp_server::Message;
+use lsp_server::Notification;
+use lsp_server::Request;
+use lsp_server::RequestId;
+use lsp_server::Response;
+use std::cell::RefCell;
+use std::collections::HashMap;
+use std::env;
+use std::fmt;
+use std::rc::Rc;
+use std::sync::Arc;
+use std::sync::RwLock;
+use std::time::Instant;
+
+type ReqHandler = fn(&mut ServerState, Response);
+type ReqQueue = lsp_server::ReqQueue<(String, Instant), ReqHandler>;
+
+pub enum Event {
+  Message(Message),
+  Task(Task),
+}
+
+impl fmt::Debug for Event {
+  fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    let debug_verbose_not =
+      |notification: &Notification, f: &mut fmt::Formatter| {
+        f.debug_struct("Notification")
+          .field("method", &notification.method)
+          .finish()
+      };
+
+    match self {
+      Event::Message(Message::Notification(notification)) => {
+        if notification_is::<lsp_types::notification::DidOpenTextDocument>(
+          notification,
+        ) || notification_is::<lsp_types::notification::DidChangeTextDocument>(
+          notification,
+        ) {
+          return debug_verbose_not(notification, f);
+        }
+      }
+      Event::Task(Task::Response(response)) => {
+        return f
+          .debug_struct("Response")
+          .field("id", &response.id)
+          .field("error", &response.error)
+          .finish();
+      }
+      _ => (),
+    }
+    match self {
+      Event::Message(it) => fmt::Debug::fmt(it, f),
+      Event::Task(it) => fmt::Debug::fmt(it, f),
+    }
+  }
+}
+
+#[derive(Eq, PartialEq, Copy, Clone)]
+pub enum Status {
+  Loading,
+  Ready,
+}
+
+impl Default for Status {
+  fn default() -> Self {
+    Status::Loading
+  }
+}
+
+#[derive(Debug)]
+pub enum Task {
+  Diagnostics((DiagnosticSource, DiagnosticVec)),
+  Response(Response),
+}
+
+#[derive(Debug, Clone)]
+pub struct DocumentData {
+  pub dependencies: Option<HashMap<String, analysis::Dependency>>,
+  pub version: Option<i32>,
+  specifier: ModuleSpecifier,
+}
+
+impl DocumentData {
+  pub fn new(
+    specifier: ModuleSpecifier,
+    version: i32,
+    source: &str,
+    maybe_import_map: Option<Rc<RefCell<ImportMap>>>,
+  ) -> Self {
+    let dependencies = if let Some((dependencies, _)) =
+      analysis::analyze_dependencies(
+        &specifier,
+        source,
+        &MediaType::from(&specifier),
+        maybe_import_map,
+      ) {
+      Some(dependencies)
+    } else {
+      None
+    };
+    Self {
+      dependencies,
+      version: Some(version),
+      specifier,
+    }
+  }
+
+  pub fn update(
+    &mut self,
+    version: i32,
+    source: &str,
+    maybe_import_map: Option<Rc<RefCell<ImportMap>>>,
+  ) {
+    self.dependencies = if let Some((dependencies, _)) =
+      analysis::analyze_dependencies(
+        &self.specifier,
+        source,
+        &MediaType::from(&self.specifier),
+        maybe_import_map,
+      ) {
+      Some(dependencies)
+    } else {
+      None
+    };
+    self.version = Some(version)
+  }
+}
+
+/// An immutable snapshot of the server state at a point in time.
+#[derive(Debug, Clone, Default)]
+pub struct ServerStateSnapshot {
+  pub config: Config,
+  pub diagnostics: DiagnosticCollection,
+  pub doc_data: HashMap<ModuleSpecifier, DocumentData>,
+  pub file_cache: Arc<RwLock<MemoryCache>>,
+  pub sources: Arc<RwLock<Sources>>,
+}
+
+pub struct ServerState {
+  pub config: Config,
+  pub diagnostics: DiagnosticCollection,
+  pub doc_data: HashMap<ModuleSpecifier, DocumentData>,
+  pub file_cache: Arc<RwLock<MemoryCache>>,
+  req_queue: ReqQueue,
+  sender: Sender<Message>,
+  pub sources: Arc<RwLock<Sources>>,
+  pub shutdown_requested: bool,
+  pub status: Status,
+  task_sender: Sender<Task>,
+  pub task_receiver: Receiver<Task>,
+  pub ts_runtime: JsRuntime,
+}
+
+impl ServerState {
+  pub fn new(sender: Sender<Message>, config: Config) -> Self {
+    let (task_sender, task_receiver) = unbounded();
+    let custom_root = env::var("DENO_DIR").map(String::into).ok();
+    let dir =
+      deno_dir::DenoDir::new(custom_root).expect("could not access DENO_DIR");
+    let location = dir.root.join("deps");
+    let sources = Sources::new(&location);
+    // TODO(@kitsonk) we need to allow displaying diagnostics here, but the
+    // current compiler snapshot sends them to stdio which would totally break
+    // the language server...
+    let ts_runtime = tsc::start(false).expect("could not start tsc");
+
+    Self {
+      config,
+      diagnostics: Default::default(),
+      doc_data: HashMap::new(),
+      file_cache: Arc::new(RwLock::new(Default::default())),
+      req_queue: Default::default(),
+      sender,
+      sources: Arc::new(RwLock::new(sources)),
+      shutdown_requested: false,
+      status: Default::default(),
+      task_receiver,
+      task_sender,
+      ts_runtime,
+    }
+  }
+
+  pub fn cancel(&mut self, request_id: RequestId) {
+    if let Some(response) = self.req_queue.incoming.cancel(request_id) {
+      self.send(response.into());
+    }
+  }
+
+  pub fn complete_request(&mut self, response: Response) {
+    let handler = self.req_queue.outgoing.complete(response.id.clone());
+    handler(self, response)
+  }
+
+  pub fn next_event(&self, inbox: &Receiver<Message>) -> Option<Event> {
+    select! {
+      recv(inbox) -> msg => msg.ok().map(Event::Message),
+      recv(self.task_receiver) -> task => Some(Event::Task(task.unwrap())),
+    }
+  }
+
+  /// Handle any changes and return a `bool` that indicates if there were
+  /// important changes to the state.
+  pub fn process_changes(&mut self) -> bool {
+    let mut file_cache = self.file_cache.write().unwrap();
+    let changed_files = file_cache.take_changes();
+    // other processing of changed files should be done here as needed
+    !changed_files.is_empty()
+  }
+
+  pub fn register_request(&mut self, request: &Request, received: Instant) {
+    self
+      .req_queue
+      .incoming
+      .register(request.id.clone(), (request.method.clone(), received));
+  }
+
+  pub fn respond(&mut self, response: Response) {
+    if let Some((_, _)) = self.req_queue.incoming.complete(response.id.clone())
+    {
+      self.send(response.into());
+    }
+  }
+
+  fn send(&mut self, message: Message) {
+    self.sender.send(message).unwrap()
+  }
+
+  pub fn send_notification<N: lsp_types::notification::Notification>(
+    &mut self,
+    params: N::Params,
+  ) {
+    let notification = Notification::new(N::METHOD.to_string(), params);
+    self.send(notification.into());
+  }
+
+  pub fn send_request<R: lsp_types::request::Request>(
+    &mut self,
+    params: R::Params,
+    handler: ReqHandler,
+  ) {
+    let request =
+      self
+        .req_queue
+        .outgoing
+        .register(R::METHOD.to_string(), params, handler);
+    self.send(request.into());
+  }
+
+  pub fn snapshot(&self) -> ServerStateSnapshot {
+    ServerStateSnapshot {
+      config: self.config.clone(),
+      diagnostics: self.diagnostics.clone(),
+      doc_data: self.doc_data.clone(),
+      file_cache: Arc::clone(&self.file_cache),
+      sources: Arc::clone(&self.sources),
+    }
+  }
+
+  pub fn spawn<F>(&mut self, task: F)
+  where
+    F: FnOnce() -> Task + Send + 'static,
+  {
+    let sender = self.task_sender.clone();
+    tokio::task::spawn_blocking(move || sender.send(task()).unwrap());
+  }
+
+  pub fn transition(&mut self, new_status: Status) {
+    self.status = new_status;
+  }
+}
diff --git a/cli/lsp/text.rs b/cli/lsp/text.rs
new file mode 100644
index 000000000..5bca534c1
--- /dev/null
+++ b/cli/lsp/text.rs
@@ -0,0 +1,514 @@
+// Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
+
+use deno_core::serde_json::json;
+use deno_core::serde_json::Value;
+use dissimilar::diff;
+use dissimilar::Chunk;
+use lsp_types::TextEdit;
+use std::ops::Bound;
+use std::ops::Range;
+use std::ops::RangeBounds;
+
+// TODO(@kitson) in general all of these text handling routines don't handle
+// JavaScript encoding in the same way and likely cause issues when trying to
+// arbitrate between chars and Unicode graphemes.  There be dragons.
+
+/// Generate a character position for the start of each line.  For example:
+///
+/// ```rust
+/// let actual = index_lines("a\nb\n");
+/// assert_eq!(actual, vec![0, 2, 4]);
+/// ```
+///
+pub fn index_lines(text: &str) -> Vec<u32> {
+  let mut indexes = vec![0_u32];
+  for (i, c) in text.chars().enumerate() {
+    if c == '\n' {
+      indexes.push((i + 1) as u32);
+    }
+  }
+  indexes
+}
+
+enum IndexValid {
+  All,
+  UpTo(u32),
+}
+
+impl IndexValid {
+  fn covers(&self, line: u32) -> bool {
+    match *self {
+      IndexValid::UpTo(to) => to > line,
+      IndexValid::All => true,
+    }
+  }
+}
+
+fn to_range(line_index: &[u32], range: lsp_types::Range) -> Range<usize> {
+  let start =
+    (line_index[range.start.line as usize] + range.start.character) as usize;
+  let end =
+    (line_index[range.end.line as usize] + range.end.character) as usize;
+  Range { start, end }
+}
+
+pub fn to_position(line_index: &[u32], char_pos: u32) -> lsp_types::Position {
+  let mut line = 0_usize;
+  let mut line_start = 0_u32;
+  for (pos, v) in line_index.iter().enumerate() {
+    if char_pos < *v {
+      break;
+    }
+    line_start = *v;
+    line = pos;
+  }
+
+  lsp_types::Position {
+    line: line as u32,
+    character: char_pos - line_start,
+  }
+}
+
+pub fn to_char_pos(line_index: &[u32], position: lsp_types::Position) -> u32 {
+  if let Some(line_start) = line_index.get(position.line as usize) {
+    line_start + position.character
+  } else {
+    0_u32
+  }
+}
+
+/// Apply a vector of document changes to the supplied string.
+pub fn apply_content_changes(
+  content: &mut String,
+  content_changes: Vec<lsp_types::TextDocumentContentChangeEvent>,
+) {
+  let mut line_index = index_lines(&content);
+  let mut index_valid = IndexValid::All;
+  for change in content_changes {
+    if let Some(range) = change.range {
+      if !index_valid.covers(range.start.line) {
+        line_index = index_lines(&content);
+      }
+      let range = to_range(&line_index, range);
+      content.replace_range(range, &change.text);
+    } else {
+      *content = change.text;
+      index_valid = IndexValid::UpTo(0);
+    }
+  }
+}
+
+/// Compare two strings and return a vector of text edit records which are
+/// supported by the Language Server Protocol.
+pub fn get_edits(a: &str, b: &str) -> Vec<TextEdit> {
+  let chunks = diff(a, b);
+  let mut text_edits = Vec::<TextEdit>::new();
+  let line_index = index_lines(a);
+  let mut iter = chunks.iter().peekable();
+  let mut a_pos = 0_u32;
+  loop {
+    let chunk = iter.next();
+    match chunk {
+      None => break,
+      Some(Chunk::Equal(e)) => {
+        a_pos += e.chars().count() as u32;
+      }
+      Some(Chunk::Delete(d)) => {
+        let start = to_position(&line_index, a_pos);
+        a_pos += d.chars().count() as u32;
+        let end = to_position(&line_index, a_pos);
+        let range = lsp_types::Range { start, end };
+        match iter.peek() {
+          Some(Chunk::Insert(i)) => {
+            iter.next();
+            text_edits.push(TextEdit {
+              range,
+              new_text: i.to_string(),
+            });
+          }
+          _ => text_edits.push(TextEdit {
+            range,
+            new_text: "".to_string(),
+          }),
+        }
+      }
+      Some(Chunk::Insert(i)) => {
+        let pos = to_position(&line_index, a_pos);
+        let range = lsp_types::Range {
+          start: pos,
+          end: pos,
+        };
+        text_edits.push(TextEdit {
+          range,
+          new_text: i.to_string(),
+        });
+      }
+    }
+  }
+
+  text_edits
+}
+
+/// Convert a difference between two strings into a change range used by the
+/// TypeScript Language Service.
+pub fn get_range_change(a: &str, b: &str) -> Value {
+  let chunks = diff(a, b);
+  let mut iter = chunks.iter().peekable();
+  let mut started = false;
+  let mut start = 0;
+  let mut end = 0;
+  let mut new_length = 0;
+  let mut equal = 0;
+  let mut a_pos = 0;
+  loop {
+    let chunk = iter.next();
+    match chunk {
+      None => break,
+      Some(Chunk::Equal(e)) => {
+        a_pos += e.chars().count();
+        equal += e.chars().count();
+      }
+      Some(Chunk::Delete(d)) => {
+        if !started {
+          start = a_pos;
+          started = true;
+          equal = 0;
+        }
+        a_pos += d.chars().count();
+        if started {
+          end = a_pos;
+          new_length += equal;
+          equal = 0;
+        }
+      }
+      Some(Chunk::Insert(i)) => {
+        if !started {
+          start = a_pos;
+          end = a_pos;
+          started = true;
+          equal = 0;
+        } else {
+          end += equal;
+        }
+        new_length += i.chars().count() + equal;
+        equal = 0;
+      }
+    }
+  }
+
+  json!({
+    "span": {
+      "start": start,
+      "length": end - start,
+    },
+    "newLength": new_length,
+  })
+}
+
+/// Provide a slice of a string based on a character range.
+pub fn slice(s: &str, range: impl RangeBounds<usize>) -> &str {
+  let start = match range.start_bound() {
+    Bound::Included(bound) | Bound::Excluded(bound) => *bound,
+    Bound::Unbounded => 0,
+  };
+  let len = match range.end_bound() {
+    Bound::Included(bound) => *bound + 1,
+    Bound::Excluded(bound) => *bound,
+    Bound::Unbounded => s.len(),
+  } - start;
+  substring(s, start, start + len)
+}
+
+/// Provide a substring based on the start and end character index positions.
+pub fn substring(s: &str, start: usize, end: usize) -> &str {
+  let len = end - start;
+  let mut char_pos = 0;
+  let mut byte_start = 0;
+  let mut it = s.chars();
+  loop {
+    if char_pos == start {
+      break;
+    }
+    if let Some(c) = it.next() {
+      char_pos += 1;
+      byte_start += c.len_utf8();
+    } else {
+      break;
+    }
+  }
+  char_pos = 0;
+  let mut byte_end = byte_start;
+  loop {
+    if char_pos == len {
+      break;
+    }
+    if let Some(c) = it.next() {
+      char_pos += 1;
+      byte_end += c.len_utf8();
+    } else {
+      break;
+    }
+  }
+  &s[byte_start..byte_end]
+}
+
+#[cfg(test)]
+mod tests {
+  use super::*;
+
+  #[test]
+  fn test_apply_content_changes() {
+    let mut content = "a\nb\nc\nd".to_string();
+    let content_changes = vec![lsp_types::TextDocumentContentChangeEvent {
+      range: Some(lsp_types::Range {
+        start: lsp_types::Position {
+          line: 1,
+          character: 0,
+        },
+        end: lsp_types::Position {
+          line: 1,
+          character: 1,
+        },
+      }),
+      range_length: Some(1),
+      text: "e".to_string(),
+    }];
+    apply_content_changes(&mut content, content_changes);
+    assert_eq!(content, "a\ne\nc\nd");
+  }
+
+  #[test]
+  fn test_get_edits() {
+    let a = "abcdefg";
+    let b = "a\nb\nchije\nfg\n";
+    let actual = get_edits(a, b);
+    assert_eq!(
+      actual,
+      vec![
+        TextEdit {
+          range: lsp_types::Range {
+            start: lsp_types::Position {
+              line: 0,
+              character: 1
+            },
+            end: lsp_types::Position {
+              line: 0,
+              character: 5
+            }
+          },
+          new_text: "\nb\nchije\n".to_string()
+        },
+        TextEdit {
+          range: lsp_types::Range {
+            start: lsp_types::Position {
+              line: 0,
+              character: 7
+            },
+            end: lsp_types::Position {
+              line: 0,
+              character: 7
+            }
+          },
+          new_text: "\n".to_string()
+        },
+      ]
+    );
+  }
+
+  #[test]
+  fn test_get_range_change() {
+    let a = "abcdefg";
+    let b = "abedcfg";
+    let actual = get_range_change(a, b);
+    assert_eq!(
+      actual,
+      json!({
+        "span": {
+          "start": 2,
+          "length": 3,
+        },
+        "newLength": 3
+      })
+    );
+
+    let a = "abfg";
+    let b = "abcdefg";
+    let actual = get_range_change(a, b);
+    assert_eq!(
+      actual,
+      json!({
+        "span": {
+          "start": 2,
+          "length": 0,
+        },
+        "newLength": 3
+      })
+    );
+
+    let a = "abcdefg";
+    let b = "abfg";
+    let actual = get_range_change(a, b);
+    assert_eq!(
+      actual,
+      json!({
+        "span": {
+          "start": 2,
+          "length": 3,
+        },
+        "newLength": 0
+      })
+    );
+
+    let a = "abcdefg";
+    let b = "abfghij";
+    let actual = get_range_change(a, b);
+    assert_eq!(
+      actual,
+      json!({
+        "span": {
+          "start": 2,
+          "length": 5,
+        },
+        "newLength": 5
+      })
+    );
+
+    let a = "abcdefghijk";
+    let b = "axcxexfxixk";
+    let actual = get_range_change(a, b);
+    assert_eq!(
+      actual,
+      json!({
+        "span": {
+          "start": 1,
+          "length": 9,
+        },
+        "newLength": 9
+      })
+    );
+
+    let a = "abcde";
+    let b = "ab(c)de";
+    let actual = get_range_change(a, b);
+    assert_eq!(
+      actual,
+      json!({
+        "span" : {
+          "start": 2,
+          "length": 1,
+        },
+        "newLength": 3
+      })
+    );
+  }
+
+  #[test]
+  fn test_index_lines() {
+    let actual = index_lines("a\nb\r\nc");
+    assert_eq!(actual, vec![0, 2, 5]);
+  }
+
+  #[test]
+  fn test_to_position() {
+    let line_index = index_lines("a\nb\r\nc\n");
+    assert_eq!(
+      to_position(&line_index, 6),
+      lsp_types::Position {
+        line: 2,
+        character: 1,
+      }
+    );
+    assert_eq!(
+      to_position(&line_index, 0),
+      lsp_types::Position {
+        line: 0,
+        character: 0,
+      }
+    );
+    assert_eq!(
+      to_position(&line_index, 3),
+      lsp_types::Position {
+        line: 1,
+        character: 1,
+      }
+    );
+  }
+
+  #[test]
+  fn test_to_position_mbc() {
+    let line_index = index_lines("y̆\n😱🦕\n🤯\n");
+    assert_eq!(
+      to_position(&line_index, 0),
+      lsp_types::Position {
+        line: 0,
+        character: 0,
+      }
+    );
+    assert_eq!(
+      to_position(&line_index, 2),
+      lsp_types::Position {
+        line: 0,
+        character: 2,
+      }
+    );
+    assert_eq!(
+      to_position(&line_index, 3),
+      lsp_types::Position {
+        line: 1,
+        character: 0,
+      }
+    );
+    assert_eq!(
+      to_position(&line_index, 4),
+      lsp_types::Position {
+        line: 1,
+        character: 1,
+      }
+    );
+    assert_eq!(
+      to_position(&line_index, 5),
+      lsp_types::Position {
+        line: 1,
+        character: 2,
+      }
+    );
+    assert_eq!(
+      to_position(&line_index, 6),
+      lsp_types::Position {
+        line: 2,
+        character: 0,
+      }
+    );
+    assert_eq!(
+      to_position(&line_index, 7),
+      lsp_types::Position {
+        line: 2,
+        character: 1,
+      }
+    );
+    assert_eq!(
+      to_position(&line_index, 8),
+      lsp_types::Position {
+        line: 3,
+        character: 0,
+      }
+    );
+  }
+
+  #[test]
+  fn test_substring() {
+    assert_eq!(substring("Deno", 1, 3), "en");
+    assert_eq!(substring("y̆y̆", 2, 4), "y̆");
+    // this doesn't work like JavaScript, as 🦕 is treated as a single char in
+    // Rust, but as two chars in JavaScript.
+    // assert_eq!(substring("🦕🦕", 2, 4), "🦕");
+  }
+
+  #[test]
+  fn test_slice() {
+    assert_eq!(slice("Deno", 1..3), "en");
+    assert_eq!(slice("Deno", 1..=3), "eno");
+    assert_eq!(slice("Deno Land", 1..), "eno Land");
+    assert_eq!(slice("Deno", ..3), "Den");
+  }
+}
diff --git a/cli/lsp/tsc.rs b/cli/lsp/tsc.rs
new file mode 100644
index 000000000..65f6ebbdb
--- /dev/null
+++ b/cli/lsp/tsc.rs
@@ -0,0 +1,1210 @@
+// Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
+
+use super::analysis::ResolvedImport;
+use super::state::ServerStateSnapshot;
+use super::text;
+use super::utils;
+
+use crate::js;
+use crate::media_type::MediaType;
+use crate::tsc::ResolveArgs;
+use crate::tsc_config::TsConfig;
+
+use deno_core::error::custom_error;
+use deno_core::error::AnyError;
+use deno_core::json_op_sync;
+use deno_core::serde::Deserialize;
+use deno_core::serde_json;
+use deno_core::serde_json::json;
+use deno_core::serde_json::Value;
+use deno_core::JsRuntime;
+use deno_core::ModuleSpecifier;
+use deno_core::OpFn;
+use deno_core::RuntimeOptions;
+use regex::Captures;
+use regex::Regex;
+use std::borrow::Cow;
+use std::collections::HashMap;
+
+/// Provide static assets for the language server.
+///
+/// TODO(@kitsonk) this should be DRY'ed up with `cli/tsc.rs` and the
+/// `cli/build.rs`
+pub fn get_asset(asset: &str) -> Option<&'static str> {
+  macro_rules! inc {
+    ($e:expr) => {
+      Some(include_str!(concat!("../dts/", $e)))
+    };
+  }
+  match asset {
+    // These are not included in the snapshot
+    "/lib.dom.d.ts" => inc!("lib.dom.d.ts"),
+    "/lib.dom.iterable.d.ts" => inc!("lib.dom.iterable.d.ts"),
+    "/lib.es6.d.ts" => inc!("lib.es6.d.ts"),
+    "/lib.es2016.full.d.ts" => inc!("lib.es2016.full.d.ts"),
+    "/lib.es2017.full.d.ts" => inc!("lib.es2017.full.d.ts"),
+    "/lib.es2018.full.d.ts" => inc!("lib.es2018.full.d.ts"),
+    "/lib.es2019.full.d.ts" => inc!("lib.es2019.full.d.ts"),
+    "/lib.es2020.full.d.ts" => inc!("lib.es2020.full.d.ts"),
+    "/lib.esnext.full.d.ts" => inc!("lib.esnext.full.d.ts"),
+    "/lib.scripthost.d.ts" => inc!("lib.scripthost.d.ts"),
+    "/lib.webworker.d.ts" => inc!("lib.webworker.d.ts"),
+    "/lib.webworker.importscripts.d.ts" => {
+      inc!("lib.webworker.importscripts.d.ts")
+    }
+    "/lib.webworker.iterable.d.ts" => inc!("lib.webworker.iterable.d.ts"),
+    // These come from op crates
+    // TODO(@kitsonk) these is even hackier than the rest of this...
+    "/lib.deno.web.d.ts" => {
+      Some(include_str!("../../op_crates/web/lib.deno_web.d.ts"))
+    }
+    "/lib.deno.fetch.d.ts" => {
+      Some(include_str!("../../op_crates/fetch/lib.deno_fetch.d.ts"))
+    }
+    // These are included in the snapshot for TypeScript, and could be retrieved
+    // from there?
+    "/lib.d.ts" => inc!("lib.d.ts"),
+    "/lib.deno.ns.d.ts" => inc!("lib.deno.ns.d.ts"),
+    "/lib.deno.shared_globals.d.ts" => inc!("lib.deno.shared_globals.d.ts"),
+    "/lib.deno.unstable.d.ts" => inc!("lib.deno.unstable.d.ts"),
+    "/lib.deno.window.d.ts" => inc!("lib.deno.window.d.ts"),
+    "/lib.deno.worker.d.ts" => inc!("lib.deno.worker.d.ts"),
+    "/lib.es5.d.ts" => inc!("lib.es5.d.ts"),
+    "/lib.es2015.collection.d.ts" => inc!("lib.es2015.collection.d.ts"),
+    "/lib.es2015.core.d.ts" => inc!("lib.es2015.core.d.ts"),
+    "/lib.es2015.d.ts" => inc!("lib.es2015.d.ts"),
+    "/lib.es2015.generator.d.ts" => inc!("lib.es2015.generator.d.ts"),
+    "/lib.es2015.iterable.d.ts" => inc!("lib.es2015.iterable.d.ts"),
+    "/lib.es2015.promise.d.ts" => inc!("lib.es2015.promise.d.ts"),
+    "/lib.es2015.proxy.d.ts" => inc!("lib.es2015.proxy.d.ts"),
+    "/lib.es2015.reflect.d.ts" => inc!("lib.es2015.reflect.d.ts"),
+    "/lib.es2015.symbol.d.ts" => inc!("lib.es2015.symbol.d.ts"),
+    "/lib.es2015.symbol.wellknown.d.ts" => {
+      inc!("lib.es2015.symbol.wellknown.d.ts")
+    }
+    "/lib.es2016.array.include.d.ts" => inc!("lib.es2016.array.include.d.ts"),
+    "/lib.es2016.d.ts" => inc!("lib.es2016.d.ts"),
+    "/lib.es2017.d.ts" => inc!("lib.es2017.d.ts"),
+    "/lib.es2017.intl.d.ts" => inc!("lib.es2017.intl.d.ts"),
+    "/lib.es2017.object.d.ts" => inc!("lib.es2017.object.d.ts"),
+    "/lib.es2017.sharedmemory.d.ts" => inc!("lib.es2017.sharedmemory.d.ts"),
+    "/lib.es2017.string.d.ts" => inc!("lib.es2017.string.d.ts"),
+    "/lib.es2017.typedarrays.d.ts" => inc!("lib.es2017.typedarrays.d.ts"),
+    "/lib.es2018.asyncgenerator.d.ts" => inc!("lib.es2018.asyncgenerator.d.ts"),
+    "/lib.es2018.asynciterable.d.ts" => inc!("lib.es2018.asynciterable.d.ts"),
+    "/lib.es2018.d.ts" => inc!("lib.es2018.d.ts"),
+    "/lib.es2018.intl.d.ts" => inc!("lib.es2018.intl.d.ts"),
+    "/lib.es2018.promise.d.ts" => inc!("lib.es2018.promise.d.ts"),
+    "/lib.es2018.regexp.d.ts" => inc!("lib.es2018.regexp.d.ts"),
+    "/lib.es2019.array.d.ts" => inc!("lib.es2019.array.d.ts"),
+    "/lib.es2019.d.ts" => inc!("lib.es2019.d.ts"),
+    "/lib.es2019.object.d.ts" => inc!("lib.es2019.object.d.ts"),
+    "/lib.es2019.string.d.ts" => inc!("lib.es2019.string.d.ts"),
+    "/lib.es2019.symbol.d.ts" => inc!("lib.es2019.symbol.d.ts"),
+    "/lib.es2020.bigint.d.ts" => inc!("lib.es2020.bigint.d.ts"),
+    "/lib.es2020.d.ts" => inc!("lib.es2020.d.ts"),
+    "/lib.es2020.intl.d.ts" => inc!("lib.es2020.intl.d.ts"),
+    "/lib.es2020.promise.d.ts" => inc!("lib.es2020.promise.d.ts"),
+    "/lib.es2020.sharedmemory.d.ts" => inc!("lib.es2020.sharedmemory.d.ts"),
+    "/lib.es2020.string.d.ts" => inc!("lib.es2020.string.d.ts"),
+    "/lib.es2020.symbol.wellknown.d.ts" => {
+      inc!("lib.es2020.symbol.wellknown.d.ts")
+    }
+    "/lib.esnext.d.ts" => inc!("lib.esnext.d.ts"),
+    "/lib.esnext.intl.d.ts" => inc!("lib.esnext.intl.d.ts"),
+    "/lib.esnext.promise.d.ts" => inc!("lib.esnext.promise.d.ts"),
+    "/lib.esnext.string.d.ts" => inc!("lib.esnext.string.d.ts"),
+    "/lib.esnext.weakref.d.ts" => inc!("lib.esnext.weakref.d.ts"),
+    _ => None,
+  }
+}
+
+fn display_parts_to_string(
+  maybe_parts: Option<Vec<SymbolDisplayPart>>,
+) -> Option<String> {
+  maybe_parts.map(|parts| {
+    parts
+      .into_iter()
+      .map(|p| p.text)
+      .collect::<Vec<String>>()
+      .join("")
+  })
+}
+
+fn get_tag_body_text(tag: &JSDocTagInfo) -> Option<String> {
+  tag.text.as_ref().map(|text| match tag.name.as_str() {
+    "example" => {
+      let caption_regex =
+        Regex::new(r"<caption>(.*?)</caption>\s*\r?\n((?:\s|\S)*)").unwrap();
+      if caption_regex.is_match(&text) {
+        caption_regex
+          .replace(text, |c: &Captures| {
+            format!("{}\n\n{}", &c[1], make_codeblock(&c[2]))
+          })
+          .to_string()
+      } else {
+        make_codeblock(text)
+      }
+    }
+    "author" => {
+      let email_match_regex = Regex::new(r"(.+)\s<([-.\w]+@[-.\w]+)>").unwrap();
+      email_match_regex
+        .replace(text, |c: &Captures| format!("{} {}", &c[1], &c[2]))
+        .to_string()
+    }
+    "default" => make_codeblock(text),
+    _ => replace_links(text),
+  })
+}
+
+fn get_tag_documentation(tag: &JSDocTagInfo) -> String {
+  match tag.name.as_str() {
+    "augments" | "extends" | "param" | "template" => {
+      if let Some(text) = &tag.text {
+        let part_regex = Regex::new(r"^(\S+)\s*-?\s*").unwrap();
+        let body: Vec<&str> = part_regex.split(&text).collect();
+        if body.len() == 3 {
+          let param = body[1];
+          let doc = body[2];
+          let label = format!("*@{}* `{}`", tag.name, param);
+          if doc.is_empty() {
+            return label;
+          }
+          if doc.contains('\n') {
+            return format!("{}  \n{}", label, replace_links(doc));
+          } else {
+            return format!("{} - {}", label, replace_links(doc));
+          }
+        }
+      }
+    }
+    _ => (),
+  }
+  let label = format!("*@{}*", tag.name);
+  let maybe_text = get_tag_body_text(tag);
+  if let Some(text) = maybe_text {
+    if text.contains('\n') {
+      format!("{}  \n{}", label, text)
+    } else {
+      format!("{} - {}", label, text)
+    }
+  } else {
+    label
+  }
+}
+
+fn make_codeblock(text: &str) -> String {
+  let codeblock_regex = Regex::new(r"^\s*[~`]{3}").unwrap();
+  if codeblock_regex.is_match(text) {
+    text.to_string()
+  } else {
+    format!("```\n{}\n```", text)
+  }
+}
+
+/// Replace JSDoc like links (`{@link http://example.com}`) with markdown links
+fn replace_links(text: &str) -> String {
+  let jsdoc_links_regex = Regex::new(r"(?i)\{@(link|linkplain|linkcode) (https?://[^ |}]+?)(?:[| ]([^{}\n]+?))?\}").unwrap();
+  jsdoc_links_regex
+    .replace_all(text, |c: &Captures| match &c[1] {
+      "linkcode" => format!(
+        "[`{}`]({})",
+        if c.get(3).is_none() {
+          &c[2]
+        } else {
+          c[3].trim()
+        },
+        &c[2]
+      ),
+      _ => format!(
+        "[{}]({})",
+        if c.get(3).is_none() {
+          &c[2]
+        } else {
+          c[3].trim()
+        },
+        &c[2]
+      ),
+    })
+    .to_string()
+}
+
+#[derive(Debug, Deserialize)]
+pub enum ScriptElementKind {
+  #[serde(rename = "")]
+  Unknown,
+  #[serde(rename = "warning")]
+  Warning,
+  #[serde(rename = "keyword")]
+  Keyword,
+  #[serde(rename = "script")]
+  ScriptElement,
+  #[serde(rename = "module")]
+  ModuleElement,
+  #[serde(rename = "class")]
+  ClassElement,
+  #[serde(rename = "local class")]
+  LocalClassElement,
+  #[serde(rename = "interface")]
+  InterfaceElement,
+  #[serde(rename = "type")]
+  TypeElement,
+  #[serde(rename = "enum")]
+  EnumElement,
+  #[serde(rename = "enum member")]
+  EnumMemberElement,
+  #[serde(rename = "var")]
+  VariableElement,
+  #[serde(rename = "local var")]
+  LocalVariableElement,
+  #[serde(rename = "function")]
+  FunctionElement,
+  #[serde(rename = "local function")]
+  LocalFunctionElement,
+  #[serde(rename = "method")]
+  MemberFunctionElement,
+  #[serde(rename = "getter")]
+  MemberGetAccessorElement,
+  #[serde(rename = "setter")]
+  MemberSetAccessorElement,
+  #[serde(rename = "property")]
+  MemberVariableElement,
+  #[serde(rename = "constructor")]
+  ConstructorImplementationElement,
+  #[serde(rename = "call")]
+  CallSignatureElement,
+  #[serde(rename = "index")]
+  IndexSignatureElement,
+  #[serde(rename = "construct")]
+  ConstructSignatureElement,
+  #[serde(rename = "parameter")]
+  ParameterElement,
+  #[serde(rename = "type parameter")]
+  TypeParameterElement,
+  #[serde(rename = "primitive type")]
+  PrimitiveType,
+  #[serde(rename = "label")]
+  Label,
+  #[serde(rename = "alias")]
+  Alias,
+  #[serde(rename = "const")]
+  ConstElement,
+  #[serde(rename = "let")]
+  LetElement,
+  #[serde(rename = "directory")]
+  Directory,
+  #[serde(rename = "external module name")]
+  ExternalModuleName,
+  #[serde(rename = "JSX attribute")]
+  JsxAttribute,
+  #[serde(rename = "string")]
+  String,
+}
+
+#[derive(Debug, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct TextSpan {
+  start: u32,
+  length: u32,
+}
+
+impl TextSpan {
+  pub fn to_range(&self, line_index: &[u32]) -> lsp_types::Range {
+    lsp_types::Range {
+      start: text::to_position(line_index, self.start),
+      end: text::to_position(line_index, self.start + self.length),
+    }
+  }
+}
+
+#[derive(Debug, Deserialize, Clone)]
+#[serde(rename_all = "camelCase")]
+pub struct SymbolDisplayPart {
+  text: String,
+  kind: String,
+}
+
+#[derive(Debug, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct JSDocTagInfo {
+  name: String,
+  text: Option<String>,
+}
+
+#[derive(Debug, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct QuickInfo {
+  kind: ScriptElementKind,
+  kind_modifiers: String,
+  text_span: TextSpan,
+  display_parts: Option<Vec<SymbolDisplayPart>>,
+  documentation: Option<Vec<SymbolDisplayPart>>,
+  tags: Option<Vec<JSDocTagInfo>>,
+}
+
+impl QuickInfo {
+  pub fn to_hover(&self, line_index: &[u32]) -> lsp_types::Hover {
+    let mut contents = Vec::<lsp_types::MarkedString>::new();
+    if let Some(display_string) =
+      display_parts_to_string(self.display_parts.clone())
+    {
+      contents.push(lsp_types::MarkedString::from_language_code(
+        "typescript".to_string(),
+        display_string,
+      ));
+    }
+    if let Some(documentation) =
+      display_parts_to_string(self.documentation.clone())
+    {
+      contents.push(lsp_types::MarkedString::from_markdown(documentation));
+    }
+    if let Some(tags) = &self.tags {
+      let tags_preview = tags
+        .iter()
+        .map(get_tag_documentation)
+        .collect::<Vec<String>>()
+        .join("  \n\n");
+      if !tags_preview.is_empty() {
+        contents.push(lsp_types::MarkedString::from_markdown(format!(
+          "\n\n{}",
+          tags_preview
+        )));
+      }
+    }
+    lsp_types::Hover {
+      contents: lsp_types::HoverContents::Array(contents),
+      range: Some(self.text_span.to_range(line_index)),
+    }
+  }
+}
+
+#[derive(Debug, Deserialize)]
+pub enum HighlightSpanKind {
+  #[serde(rename = "none")]
+  None,
+  #[serde(rename = "definition")]
+  Definition,
+  #[serde(rename = "reference")]
+  Reference,
+  #[serde(rename = "writtenReference")]
+  WrittenReference,
+}
+
+#[derive(Debug, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct HighlightSpan {
+  file_name: Option<String>,
+  is_in_string: Option<bool>,
+  text_span: TextSpan,
+  context_span: Option<TextSpan>,
+  kind: HighlightSpanKind,
+}
+
+#[derive(Debug, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct DefinitionInfo {
+  kind: ScriptElementKind,
+  name: String,
+  container_kind: Option<ScriptElementKind>,
+  container_name: Option<String>,
+  text_span: TextSpan,
+  pub file_name: String,
+  original_text_span: Option<TextSpan>,
+  original_file_name: Option<String>,
+  context_span: Option<TextSpan>,
+  original_context_span: Option<TextSpan>,
+}
+
+#[derive(Debug, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct DefinitionInfoAndBoundSpan {
+  pub definitions: Option<Vec<DefinitionInfo>>,
+  text_span: TextSpan,
+}
+
+impl DefinitionInfoAndBoundSpan {
+  pub fn to_definition<F>(
+    &self,
+    line_index: &[u32],
+    mut index_provider: F,
+  ) -> Option<lsp_types::GotoDefinitionResponse>
+  where
+    F: FnMut(ModuleSpecifier) -> Vec<u32>,
+  {
+    if let Some(definitions) = &self.definitions {
+      let location_links = definitions
+        .iter()
+        .map(|di| {
+          let target_specifier =
+            ModuleSpecifier::resolve_url(&di.file_name).unwrap();
+          let target_line_index = index_provider(target_specifier);
+          let target_uri = utils::normalize_file_name(&di.file_name).unwrap();
+          let (target_range, target_selection_range) =
+            if let Some(context_span) = &di.context_span {
+              (
+                context_span.to_range(&target_line_index),
+                di.text_span.to_range(&target_line_index),
+              )
+            } else {
+              (
+                di.text_span.to_range(&target_line_index),
+                di.text_span.to_range(&target_line_index),
+              )
+            };
+          lsp_types::LocationLink {
+            origin_selection_range: Some(self.text_span.to_range(line_index)),
+            target_uri,
+            target_range,
+            target_selection_range,
+          }
+        })
+        .collect();
+
+      Some(lsp_types::GotoDefinitionResponse::Link(location_links))
+    } else {
+      None
+    }
+  }
+}
+
+#[derive(Debug, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct DocumentHighlights {
+  file_name: String,
+  highlight_spans: Vec<HighlightSpan>,
+}
+
+impl DocumentHighlights {
+  pub fn to_highlight(
+    &self,
+    line_index: &[u32],
+  ) -> Vec<lsp_types::DocumentHighlight> {
+    self
+      .highlight_spans
+      .iter()
+      .map(|hs| lsp_types::DocumentHighlight {
+        range: hs.text_span.to_range(line_index),
+        kind: match hs.kind {
+          HighlightSpanKind::WrittenReference => {
+            Some(lsp_types::DocumentHighlightKind::Write)
+          }
+          _ => Some(lsp_types::DocumentHighlightKind::Read),
+        },
+      })
+      .collect()
+  }
+}
+
+#[derive(Debug, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct ReferenceEntry {
+  is_write_access: bool,
+  pub is_definition: bool,
+  is_in_string: Option<bool>,
+  text_span: TextSpan,
+  pub file_name: String,
+  original_text_span: Option<TextSpan>,
+  original_file_name: Option<String>,
+  context_span: Option<TextSpan>,
+  original_context_span: Option<TextSpan>,
+}
+
+impl ReferenceEntry {
+  pub fn to_location(&self, line_index: &[u32]) -> lsp_types::Location {
+    let uri = utils::normalize_file_name(&self.file_name).unwrap();
+    lsp_types::Location {
+      uri,
+      range: self.text_span.to_range(line_index),
+    }
+  }
+}
+
+#[derive(Debug, Clone, Deserialize)]
+struct Response {
+  id: usize,
+  data: Value,
+}
+
+struct State<'a> {
+  last_id: usize,
+  response: Option<Response>,
+  server_state: ServerStateSnapshot,
+  snapshots: HashMap<(Cow<'a, str>, Cow<'a, str>), String>,
+}
+
+impl<'a> State<'a> {
+  fn new(server_state: ServerStateSnapshot) -> Self {
+    Self {
+      last_id: 1,
+      response: None,
+      server_state,
+      snapshots: Default::default(),
+    }
+  }
+}
+
+/// If a snapshot is missing from the state cache, add it.
+fn cache_snapshot(
+  state: &mut State,
+  specifier: String,
+  version: String,
+) -> Result<(), AnyError> {
+  if !state
+    .snapshots
+    .contains_key(&(specifier.clone().into(), version.clone().into()))
+  {
+    let s = ModuleSpecifier::resolve_url(&specifier)?;
+    let file_cache = state.server_state.file_cache.read().unwrap();
+    let file_id = file_cache.lookup(&s).unwrap();
+    let content = file_cache.get_contents(file_id)?;
+    state
+      .snapshots
+      .insert((specifier.into(), version.into()), content);
+  }
+  Ok(())
+}
+
+fn op<F>(op_fn: F) -> Box<OpFn>
+where
+  F: Fn(&mut State, Value) -> Result<Value, AnyError> + 'static,
+{
+  json_op_sync(move |s, args, _bufs| {
+    let state = s.borrow_mut::<State>();
+    op_fn(state, args)
+  })
+}
+
+#[derive(Debug, Deserialize)]
+#[serde(rename_all = "camelCase")]
+struct SourceSnapshotArgs {
+  specifier: String,
+  version: String,
+}
+
+/// The language service is dropping a reference to a source file snapshot, and
+/// we can drop our version of that document.
+fn dispose(state: &mut State, args: Value) -> Result<Value, AnyError> {
+  let v: SourceSnapshotArgs = serde_json::from_value(args)?;
+  state
+    .snapshots
+    .remove(&(v.specifier.into(), v.version.into()));
+  Ok(json!(true))
+}
+
+#[derive(Debug, Deserialize)]
+#[serde(rename_all = "camelCase")]
+struct GetChangeRangeArgs {
+  specifier: String,
+  old_length: u32,
+  old_version: String,
+  version: String,
+}
+
+/// The language service wants to compare an old snapshot with a new snapshot to
+/// determine what source hash changed.
+fn get_change_range(state: &mut State, args: Value) -> Result<Value, AnyError> {
+  let v: GetChangeRangeArgs = serde_json::from_value(args.clone())?;
+  cache_snapshot(state, v.specifier.clone(), v.version.clone())?;
+  if let Some(current) = state
+    .snapshots
+    .get(&(v.specifier.clone().into(), v.version.into()))
+  {
+    if let Some(prev) = state
+      .snapshots
+      .get(&(v.specifier.clone().into(), v.old_version.clone().into()))
+    {
+      Ok(text::get_range_change(prev, current))
+    } else {
+      // when a local file is opened up in the editor, the compiler might
+      // already have a snapshot of it in memory, and will request it, but we
+      // now are working off in memory versions of the document, and so need
+      // to tell tsc to reset the whole document
+      Ok(json!({
+        "span": {
+          "start": 0,
+          "length": v.old_length,
+        },
+        "newLength": current.chars().count(),
+      }))
+    }
+  } else {
+    Err(custom_error(
+      "MissingSnapshot",
+      format!(
+        "The current snapshot version is missing.\n  Args: \"{}\"",
+        args
+      ),
+    ))
+  }
+}
+
+fn get_length(state: &mut State, args: Value) -> Result<Value, AnyError> {
+  let v: SourceSnapshotArgs = serde_json::from_value(args)?;
+  let specifier = ModuleSpecifier::resolve_url(&v.specifier)?;
+  if state.server_state.doc_data.contains_key(&specifier) {
+    cache_snapshot(state, v.specifier.clone(), v.version.clone())?;
+    let content = state
+      .snapshots
+      .get(&(v.specifier.into(), v.version.into()))
+      .unwrap();
+    Ok(json!(content.chars().count()))
+  } else {
+    let mut sources = state.server_state.sources.write().unwrap();
+    Ok(json!(sources.get_length(&specifier).unwrap()))
+  }
+}
+
+#[derive(Debug, Deserialize)]
+#[serde(rename_all = "camelCase")]
+struct GetTextArgs {
+  specifier: String,
+  version: String,
+  start: usize,
+  end: usize,
+}
+
+fn get_text(state: &mut State, args: Value) -> Result<Value, AnyError> {
+  let v: GetTextArgs = serde_json::from_value(args)?;
+  let specifier = ModuleSpecifier::resolve_url(&v.specifier)?;
+  let content = if state.server_state.doc_data.contains_key(&specifier) {
+    cache_snapshot(state, v.specifier.clone(), v.version.clone())?;
+    state
+      .snapshots
+      .get(&(v.specifier.into(), v.version.into()))
+      .unwrap()
+      .clone()
+  } else {
+    let mut sources = state.server_state.sources.write().unwrap();
+    sources.get_text(&specifier).unwrap()
+  };
+  Ok(json!(text::slice(&content, v.start..v.end)))
+}
+
+fn resolve(state: &mut State, args: Value) -> Result<Value, AnyError> {
+  let v: ResolveArgs = serde_json::from_value(args)?;
+  let mut resolved = Vec::<Option<(String, String)>>::new();
+  let referrer = ModuleSpecifier::resolve_url(&v.base)?;
+  let mut sources = if let Ok(sources) = state.server_state.sources.write() {
+    sources
+  } else {
+    return Err(custom_error("Deadlock", "deadlock locking sources"));
+  };
+
+  if let Some(doc_data) = state.server_state.doc_data.get(&referrer) {
+    if let Some(dependencies) = &doc_data.dependencies {
+      for specifier in &v.specifiers {
+        if specifier.starts_with("asset:///") {
+          resolved.push(Some((
+            specifier.clone(),
+            MediaType::from(specifier).as_ts_extension(),
+          )))
+        } else if let Some(dependency) = dependencies.get(specifier) {
+          let resolved_import =
+            if let Some(resolved_import) = &dependency.maybe_type {
+              resolved_import.clone()
+            } else if let Some(resolved_import) = &dependency.maybe_code {
+              resolved_import.clone()
+            } else {
+              ResolvedImport::Err("missing dependency".to_string())
+            };
+          if let ResolvedImport::Resolved(resolved_specifier) = resolved_import
+          {
+            let media_type = if let Some(media_type) =
+              sources.get_media_type(&resolved_specifier)
+            {
+              media_type
+            } else {
+              MediaType::from(&resolved_specifier)
+            };
+            resolved.push(Some((
+              resolved_specifier.to_string(),
+              media_type.as_ts_extension(),
+            )));
+          } else {
+            resolved.push(None);
+          }
+        }
+      }
+    }
+  } else if sources.contains(&referrer) {
+    for specifier in &v.specifiers {
+      if let Some((resolved_specifier, media_type)) =
+        sources.resolve_import(specifier, &referrer)
+      {
+        resolved.push(Some((
+          resolved_specifier.to_string(),
+          media_type.as_ts_extension(),
+        )));
+      } else {
+        resolved.push(None);
+      }
+    }
+  } else {
+    return Err(custom_error(
+      "NotFound",
+      "the referring specifier is unexpectedly missing",
+    ));
+  }
+
+  Ok(json!(resolved))
+}
+
+fn respond(state: &mut State, args: Value) -> Result<Value, AnyError> {
+  state.response = Some(serde_json::from_value(args)?);
+  Ok(json!(true))
+}
+
+fn script_names(state: &mut State, _args: Value) -> Result<Value, AnyError> {
+  let script_names: Vec<&ModuleSpecifier> =
+    state.server_state.doc_data.keys().collect();
+  Ok(json!(script_names))
+}
+
+#[derive(Debug, Deserialize)]
+#[serde(rename_all = "camelCase")]
+struct ScriptVersionArgs {
+  specifier: String,
+}
+
+fn script_version(state: &mut State, args: Value) -> Result<Value, AnyError> {
+  let v: ScriptVersionArgs = serde_json::from_value(args)?;
+  let specifier = ModuleSpecifier::resolve_url(&v.specifier)?;
+  let maybe_doc_data = state.server_state.doc_data.get(&specifier);
+  if let Some(doc_data) = maybe_doc_data {
+    if let Some(version) = doc_data.version {
+      return Ok(json!(version.to_string()));
+    }
+  } else {
+    let mut sources = state.server_state.sources.write().unwrap();
+    if let Some(version) = sources.get_script_version(&specifier) {
+      return Ok(json!(version));
+    }
+  }
+
+  Ok(json!(None::<String>))
+}
+
+/// Create and setup a JsRuntime based on a snapshot. It is expected that the
+/// supplied snapshot is an isolate that contains the TypeScript language
+/// server.
+pub fn start(debug: bool) -> Result<JsRuntime, AnyError> {
+  let mut runtime = JsRuntime::new(RuntimeOptions {
+    startup_snapshot: Some(js::compiler_isolate_init()),
+    ..Default::default()
+  });
+
+  {
+    let op_state = runtime.op_state();
+    let mut op_state = op_state.borrow_mut();
+    op_state.put(State::new(ServerStateSnapshot::default()));
+  }
+
+  runtime.register_op("op_dispose", op(dispose));
+  runtime.register_op("op_get_change_range", op(get_change_range));
+  runtime.register_op("op_get_length", op(get_length));
+  runtime.register_op("op_get_text", op(get_text));
+  runtime.register_op("op_resolve", op(resolve));
+  runtime.register_op("op_respond", op(respond));
+  runtime.register_op("op_script_names", op(script_names));
+  runtime.register_op("op_script_version", op(script_version));
+
+  let init_config = json!({ "debug": debug });
+  let init_src = format!("globalThis.serverInit({});", init_config);
+
+  runtime.execute("[native code]", &init_src)?;
+  Ok(runtime)
+}
+
+/// Methods that are supported by the Language Service in the compiler isolate.
+pub enum RequestMethod {
+  /// Configure the compilation settings for the server.
+  Configure(TsConfig),
+  /// Return semantic diagnostics for given file.
+  GetSemanticDiagnostics(ModuleSpecifier),
+  /// Returns suggestion diagnostics for given file.
+  GetSuggestionDiagnostics(ModuleSpecifier),
+  /// Return syntactic diagnostics for a given file.
+  GetSyntacticDiagnostics(ModuleSpecifier),
+  /// Return quick info at position (hover information).
+  GetQuickInfo((ModuleSpecifier, u32)),
+  /// Return document highlights at position.
+  GetDocumentHighlights((ModuleSpecifier, u32, Vec<ModuleSpecifier>)),
+  /// Get document references for a specific position.
+  GetReferences((ModuleSpecifier, u32)),
+  /// Get declaration information for a specific position.
+  GetDefinition((ModuleSpecifier, u32)),
+}
+
+impl RequestMethod {
+  pub fn to_value(&self, id: usize) -> Value {
+    match self {
+      RequestMethod::Configure(config) => json!({
+        "id": id,
+        "method": "configure",
+        "compilerOptions": config,
+      }),
+      RequestMethod::GetSemanticDiagnostics(specifier) => json!({
+        "id": id,
+        "method": "getSemanticDiagnostics",
+        "specifier": specifier,
+      }),
+      RequestMethod::GetSuggestionDiagnostics(specifier) => json!({
+        "id": id,
+        "method": "getSuggestionDiagnostics",
+        "specifier": specifier,
+      }),
+      RequestMethod::GetSyntacticDiagnostics(specifier) => json!({
+        "id": id,
+        "method": "getSyntacticDiagnostics",
+        "specifier": specifier,
+      }),
+      RequestMethod::GetQuickInfo((specifier, position)) => json!({
+        "id": id,
+        "method": "getQuickInfo",
+        "specifier": specifier,
+        "position": position,
+      }),
+      RequestMethod::GetDocumentHighlights((
+        specifier,
+        position,
+        files_to_search,
+      )) => json!({
+        "id": id,
+        "method": "getDocumentHighlights",
+        "specifier": specifier,
+        "position": position,
+        "filesToSearch": files_to_search,
+      }),
+      RequestMethod::GetReferences((specifier, position)) => json!({
+        "id": id,
+        "method": "getReferences",
+        "specifier": specifier,
+        "position": position,
+      }),
+      RequestMethod::GetDefinition((specifier, position)) => json!({
+        "id": id,
+        "method": "getDefinition",
+        "specifier": specifier,
+        "position": position,
+      }),
+    }
+  }
+}
+
+/// Send a request into a runtime and return the JSON value of the response.
+pub fn request(
+  runtime: &mut JsRuntime,
+  server_state: &ServerStateSnapshot,
+  method: RequestMethod,
+) -> Result<Value, AnyError> {
+  let id = {
+    let op_state = runtime.op_state();
+    let mut op_state = op_state.borrow_mut();
+    let state = op_state.borrow_mut::<State>();
+    state.server_state = server_state.clone();
+    state.last_id += 1;
+    state.last_id
+  };
+  let request_params = method.to_value(id);
+  let request_src = format!("globalThis.serverRequest({});", request_params);
+  runtime.execute("[native_code]", &request_src)?;
+
+  let op_state = runtime.op_state();
+  let mut op_state = op_state.borrow_mut();
+  let state = op_state.borrow_mut::<State>();
+
+  if let Some(response) = state.response.clone() {
+    state.response = None;
+    Ok(response.data)
+  } else {
+    Err(custom_error(
+      "RequestError",
+      "The response was not received for the request.",
+    ))
+  }
+}
+
+#[cfg(test)]
+mod tests {
+  use super::super::memory_cache::MemoryCache;
+  use super::super::state::DocumentData;
+  use super::*;
+  use std::collections::HashMap;
+  use std::sync::Arc;
+  use std::sync::RwLock;
+
+  fn mock_server_state(sources: Vec<(&str, &str, i32)>) -> ServerStateSnapshot {
+    let mut doc_data = HashMap::new();
+    let mut file_cache = MemoryCache::default();
+    for (specifier, content, version) in sources {
+      let specifier = ModuleSpecifier::resolve_url(specifier)
+        .expect("failed to create specifier");
+      doc_data.insert(
+        specifier.clone(),
+        DocumentData::new(specifier.clone(), version, content, None),
+      );
+      file_cache.set_contents(specifier, Some(content.as_bytes().to_vec()));
+    }
+    let file_cache = Arc::new(RwLock::new(file_cache));
+    ServerStateSnapshot {
+      config: Default::default(),
+      diagnostics: Default::default(),
+      doc_data,
+      file_cache,
+      sources: Default::default(),
+    }
+  }
+
+  fn setup(
+    debug: bool,
+    config: Value,
+    sources: Vec<(&str, &str, i32)>,
+  ) -> (JsRuntime, ServerStateSnapshot) {
+    let server_state = mock_server_state(sources.clone());
+    let mut runtime = start(debug).expect("could not start server");
+    let ts_config = TsConfig::new(config);
+    assert_eq!(
+      request(
+        &mut runtime,
+        &server_state,
+        RequestMethod::Configure(ts_config)
+      )
+      .expect("failed request"),
+      json!(true)
+    );
+    (runtime, server_state)
+  }
+
+  #[test]
+  fn test_replace_links() {
+    let actual = replace_links(r"test {@link http://deno.land/x/mod.ts} test");
+    assert_eq!(
+      actual,
+      r"test [http://deno.land/x/mod.ts](http://deno.land/x/mod.ts) test"
+    );
+    let actual =
+      replace_links(r"test {@link http://deno.land/x/mod.ts a link} test");
+    assert_eq!(actual, r"test [a link](http://deno.land/x/mod.ts) test");
+    let actual =
+      replace_links(r"test {@linkcode http://deno.land/x/mod.ts a link} test");
+    assert_eq!(actual, r"test [`a link`](http://deno.land/x/mod.ts) test");
+  }
+
+  #[test]
+  fn test_project_configure() {
+    setup(
+      false,
+      json!({
+        "target": "esnext",
+        "module": "esnext",
+        "noEmit": true,
+      }),
+      vec![],
+    );
+  }
+
+  #[test]
+  fn test_project_reconfigure() {
+    let (mut runtime, server_state) = setup(
+      false,
+      json!({
+        "target": "esnext",
+        "module": "esnext",
+        "noEmit": true,
+      }),
+      vec![],
+    );
+    let ts_config = TsConfig::new(json!({
+      "target": "esnext",
+      "module": "esnext",
+      "noEmit": true,
+      "lib": ["deno.ns", "deno.worker"]
+    }));
+    let result = request(
+      &mut runtime,
+      &server_state,
+      RequestMethod::Configure(ts_config),
+    );
+    assert!(result.is_ok());
+    let response = result.unwrap();
+    assert_eq!(response, json!(true));
+  }
+
+  #[test]
+  fn test_get_semantic_diagnostics() {
+    let (mut runtime, server_state) = setup(
+      false,
+      json!({
+        "target": "esnext",
+        "module": "esnext",
+        "noEmit": true,
+      }),
+      vec![("file:///a.ts", r#"console.log("hello deno");"#, 1)],
+    );
+    let specifier = ModuleSpecifier::resolve_url("file:///a.ts")
+      .expect("could not resolve url");
+    let result = request(
+      &mut runtime,
+      &server_state,
+      RequestMethod::GetSemanticDiagnostics(specifier),
+    );
+    assert!(result.is_ok());
+    let response = result.unwrap();
+    assert_eq!(
+      response,
+      json!([
+        {
+          "start": {
+            "line": 0,
+            "character": 0,
+          },
+          "end": {
+            "line": 0,
+            "character": 7
+          },
+          "fileName": "file:///a.ts",
+          "messageText": "Cannot find name 'console'. Do you need to change your target library? Try changing the `lib` compiler option to include 'dom'.",
+          "sourceLine": "console.log(\"hello deno\");",
+          "category": 1,
+          "code": 2584
+        }
+      ])
+    );
+  }
+
+  #[test]
+  fn test_module_resolution() {
+    let (mut runtime, server_state) = setup(
+      false,
+      json!({
+        "target": "esnext",
+        "module": "esnext",
+        "lib": ["deno.ns", "deno.window"],
+        "noEmit": true,
+      }),
+      vec![(
+        "file:///a.ts",
+        r#"
+        import { B } from "https://deno.land/x/b/mod.ts";
+
+        const b = new B();
+
+        console.log(b);
+      "#,
+        1,
+      )],
+    );
+    let specifier = ModuleSpecifier::resolve_url("file:///a.ts")
+      .expect("could not resolve url");
+    let result = request(
+      &mut runtime,
+      &server_state,
+      RequestMethod::GetSemanticDiagnostics(specifier),
+    );
+    assert!(result.is_ok());
+    let response = result.unwrap();
+    assert_eq!(response, json!([]));
+  }
+
+  #[test]
+  fn test_bad_module_specifiers() {
+    let (mut runtime, server_state) = setup(
+      false,
+      json!({
+        "target": "esnext",
+        "module": "esnext",
+        "lib": ["deno.ns", "deno.window"],
+        "noEmit": true,
+      }),
+      vec![(
+        "file:///a.ts",
+        r#"
+        import { A } from ".";
+        "#,
+        1,
+      )],
+    );
+    let specifier = ModuleSpecifier::resolve_url("file:///a.ts")
+      .expect("could not resolve url");
+    let result = request(
+      &mut runtime,
+      &server_state,
+      RequestMethod::GetSyntacticDiagnostics(specifier),
+    );
+    assert!(result.is_ok());
+    let response = result.unwrap();
+    assert_eq!(response, json!([]));
+  }
+
+  #[test]
+  fn test_remote_modules() {
+    let (mut runtime, server_state) = setup(
+      false,
+      json!({
+        "target": "esnext",
+        "module": "esnext",
+        "lib": ["deno.ns", "deno.window"],
+        "noEmit": true,
+      }),
+      vec![(
+        "file:///a.ts",
+        r#"
+        import { B } from "https://deno.land/x/b/mod.ts";
+
+        const b = new B();
+
+        console.log(b);
+      "#,
+        1,
+      )],
+    );
+    let specifier = ModuleSpecifier::resolve_url("file:///a.ts")
+      .expect("could not resolve url");
+    let result = request(
+      &mut runtime,
+      &server_state,
+      RequestMethod::GetSyntacticDiagnostics(specifier),
+    );
+    assert!(result.is_ok());
+    let response = result.unwrap();
+    assert_eq!(response, json!([]));
+  }
+
+  #[test]
+  fn test_partial_modules() {
+    let (mut runtime, server_state) = setup(
+      false,
+      json!({
+        "target": "esnext",
+        "module": "esnext",
+        "lib": ["deno.ns", "deno.window"],
+        "noEmit": true,
+      }),
+      vec![(
+        "file:///a.ts",
+        r#"
+        import {
+          Application,
+          Context,
+          Router,
+          Status,
+        } from "https://deno.land/x/oak@v6.3.2/mod.ts";
+        
+        import * as test from
+      "#,
+        1,
+      )],
+    );
+    let specifier = ModuleSpecifier::resolve_url("file:///a.ts")
+      .expect("could not resolve url");
+    let result = request(
+      &mut runtime,
+      &server_state,
+      RequestMethod::GetSyntacticDiagnostics(specifier),
+    );
+    println!("{:?}", result);
+    // assert!(result.is_ok());
+    // let response = result.unwrap();
+    // assert_eq!(response, json!([]));
+  }
+}
diff --git a/cli/lsp/utils.rs b/cli/lsp/utils.rs
new file mode 100644
index 000000000..0c3d5a635
--- /dev/null
+++ b/cli/lsp/utils.rs
@@ -0,0 +1,114 @@
+// Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
+
+use deno_core::error::custom_error;
+use deno_core::error::AnyError;
+use deno_core::serde_json::Value;
+use deno_core::url::Position;
+use deno_core::url::Url;
+use deno_core::ModuleSpecifier;
+use lsp_server::Notification;
+use serde::de::DeserializeOwned;
+use std::error::Error;
+use std::fmt;
+
+// TODO(@kitsonk) support actually supporting cancellation requests from the
+// client.
+
+pub struct Canceled {
+  _private: (),
+}
+
+impl Canceled {
+  #[allow(unused)]
+  pub fn new() -> Self {
+    Self { _private: () }
+  }
+
+  #[allow(unused)]
+  pub fn throw() -> ! {
+    std::panic::resume_unwind(Box::new(Canceled::new()))
+  }
+}
+
+impl fmt::Display for Canceled {
+  fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    write!(f, "cancelled")
+  }
+}
+
+impl fmt::Debug for Canceled {
+  fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    write!(f, "Canceled")
+  }
+}
+
+impl Error for Canceled {}
+
+pub fn from_json<T: DeserializeOwned>(
+  what: &'static str,
+  json: Value,
+) -> Result<T, AnyError> {
+  let response = T::deserialize(&json).map_err(|err| {
+    custom_error(
+      "DeserializeFailed",
+      format!("Failed to deserialize {}: {}; {}", what, err, json),
+    )
+  })?;
+  Ok(response)
+}
+
+pub fn is_canceled(e: &(dyn Error + 'static)) -> bool {
+  e.downcast_ref::<Canceled>().is_some()
+}
+
+pub fn notification_is<N: lsp_types::notification::Notification>(
+  notification: &Notification,
+) -> bool {
+  notification.method == N::METHOD
+}
+
+/// Normalizes a file name returned from the TypeScript compiler into a URI that
+/// should be sent by the language server to the client.
+pub fn normalize_file_name(file_name: &str) -> Result<Url, AnyError> {
+  let specifier_str = if file_name.starts_with("file://") {
+    file_name.to_string()
+  } else {
+    format!("deno:///{}", file_name.replacen("://", "/", 1))
+  };
+  Url::parse(&specifier_str).map_err(|err| err.into())
+}
+
+/// Normalize URLs from the client, where "virtual" `deno:///` URLs are
+/// converted into proper module specifiers.
+pub fn normalize_url(url: Url) -> ModuleSpecifier {
+  if url.scheme() == "deno"
+    && (url.path().starts_with("/http") || url.path().starts_with("/asset"))
+  {
+    let specifier_str = url[Position::BeforePath..]
+      .replacen("/", "", 1)
+      .replacen("/", "://", 1);
+    if let Ok(specifier) =
+      percent_encoding::percent_decode_str(&specifier_str).decode_utf8()
+    {
+      if let Ok(specifier) = ModuleSpecifier::resolve_url(&specifier) {
+        return specifier;
+      }
+    }
+  }
+  ModuleSpecifier::from(url)
+}
+
+#[cfg(test)]
+mod tests {
+  use super::*;
+
+  #[test]
+  fn test_normalize_url() {
+    let fixture = Url::parse("deno:///https/deno.land/x/mod.ts").unwrap();
+    let actual = normalize_url(fixture);
+    assert_eq!(
+      actual,
+      ModuleSpecifier::resolve_url("https://deno.land/x/mod.ts").unwrap()
+    );
+  }
+}
diff --git a/cli/main.rs b/cli/main.rs
index e297d0c4c..2e40df66b 100644
--- a/cli/main.rs
+++ b/cli/main.rs
@@ -28,6 +28,7 @@ mod info;
 mod inspector;
 mod js;
 mod lockfile;
+mod lsp;
 mod media_type;
 mod metrics;
 mod module_graph;
@@ -258,6 +259,10 @@ async fn install_command(
   tools::installer::install(flags, &module_url, args, name, root, force)
 }
 
+async fn language_server_command() -> Result<(), AnyError> {
+  lsp::start()
+}
+
 async fn lint_command(
   flags: Flags,
   files: Vec<PathBuf>,
@@ -992,6 +997,7 @@ fn get_subcommand(
     } => {
       install_command(flags, module_url, args, name, root, force).boxed_local()
     }
+    DenoSubcommand::LanguageServer => language_server_command().boxed_local(),
     DenoSubcommand::Lint {
       files,
       rules,
diff --git a/cli/module_graph.rs b/cli/module_graph.rs
index 8c6f69552..4144ee5ee 100644
--- a/cli/module_graph.rs
+++ b/cli/module_graph.rs
@@ -31,6 +31,8 @@ use crate::AnyError;
 use deno_core::error::Context;
 use deno_core::futures::stream::FuturesUnordered;
 use deno_core::futures::stream::StreamExt;
+use deno_core::serde::Deserialize;
+use deno_core::serde::Deserializer;
 use deno_core::serde::Serialize;
 use deno_core::serde::Serializer;
 use deno_core::serde_json::json;
@@ -38,8 +40,6 @@ use deno_core::serde_json::Value;
 use deno_core::ModuleResolutionError;
 use deno_core::ModuleSpecifier;
 use regex::Regex;
-use serde::Deserialize;
-use serde::Deserializer;
 use std::cell::RefCell;
 use std::collections::HashSet;
 use std::collections::{BTreeSet, HashMap};
@@ -182,14 +182,14 @@ impl swc_bundler::Load for BundleLoader<'_> {
 
 /// An enum which represents the parsed out values of references in source code.
 #[derive(Debug, Clone, Eq, PartialEq)]
-enum TypeScriptReference {
+pub enum TypeScriptReference {
   Path(String),
   Types(String),
 }
 
 /// Determine if a comment contains a triple slash reference and optionally
 /// return its kind and value.
-fn parse_ts_reference(comment: &str) -> Option<TypeScriptReference> {
+pub fn parse_ts_reference(comment: &str) -> Option<TypeScriptReference> {
   if !TRIPLE_SLASH_REFERENCE_RE.is_match(comment) {
     None
   } else if let Some(captures) = PATH_REFERENCE_RE.captures(comment) {
@@ -207,7 +207,7 @@ fn parse_ts_reference(comment: &str) -> Option<TypeScriptReference> {
 
 /// Determine if a comment contains a `@deno-types` pragma and optionally return
 /// its value.
-fn parse_deno_types(comment: &str) -> Option<String> {
+pub fn parse_deno_types(comment: &str) -> Option<String> {
   if let Some(captures) = DENO_TYPES_RE.captures(comment) {
     if let Some(m) = captures.get(1) {
       Some(m.as_str().to_string())
@@ -230,8 +230,8 @@ fn get_version(source: &str, version: &str, config: &[u8]) -> String {
 
 /// A logical representation of a module within a graph.
 #[derive(Debug, Clone)]
-struct Module {
-  dependencies: DependencyMap,
+pub struct Module {
+  pub dependencies: DependencyMap,
   is_dirty: bool,
   is_parsed: bool,
   maybe_emit: Option<Emit>,
diff --git a/cli/tests/integration_tests.rs b/cli/tests/integration_tests.rs
index aca2df99c..42172a771 100644
--- a/cli/tests/integration_tests.rs
+++ b/cli/tests/integration_tests.rs
@@ -914,7 +914,7 @@ fn ts_reload() {
   assert!(std::str::from_utf8(&output.stdout)
     .unwrap()
     .trim()
-    .contains("\"host.writeFile(\\\"deno://002_hello.js\\\")\""));
+    .contains("host.writeFile(\"deno://002_hello.js\")"));
 }
 
 #[test]
diff --git a/cli/tests/lsp/did_open_notification.json b/cli/tests/lsp/did_open_notification.json
new file mode 100644
index 000000000..04f12a7b3
--- /dev/null
+++ b/cli/tests/lsp/did_open_notification.json
@@ -0,0 +1,12 @@
+{
+  "jsonrpc": "2.0",
+  "method": "textDocument/didOpen",
+  "params": {
+    "textDocument": {
+      "uri": "file:///a/file.ts",
+      "languageId": "typescript",
+      "version": 1,
+      "text": "console.log(Deno.args);\n"
+    }
+  }
+}
diff --git a/cli/tests/lsp/exit_notification.json b/cli/tests/lsp/exit_notification.json
new file mode 100644
index 000000000..799a0d1d5
--- /dev/null
+++ b/cli/tests/lsp/exit_notification.json
@@ -0,0 +1,5 @@
+{
+  "jsonrpc": "2.0",
+  "method": "exit",
+  "params": null
+}
diff --git a/cli/tests/lsp/hover_request.json b/cli/tests/lsp/hover_request.json
new file mode 100644
index 000000000..f12bd52df
--- /dev/null
+++ b/cli/tests/lsp/hover_request.json
@@ -0,0 +1,14 @@
+{
+  "jsonrpc": "2.0",
+  "id": 2,
+  "method": "textDocument/hover",
+  "params": {
+    "textDocument": {
+      "uri": "file:///a/file.ts"
+    },
+    "position": {
+      "line": 0,
+      "character": 19
+    }
+  }
+}
diff --git a/cli/tests/lsp/initialize_request.json b/cli/tests/lsp/initialize_request.json
new file mode 100644
index 000000000..960420bfd
--- /dev/null
+++ b/cli/tests/lsp/initialize_request.json
@@ -0,0 +1,23 @@
+{
+  "jsonrpc": "2.0",
+  "id": 1,
+  "method": "initialize",
+  "params": {
+    "processId": 0,
+    "clientInfo": {
+      "name": "test-harness",
+      "version": "1.0.0"
+    },
+    "rootUri": null,
+    "capabilities": {
+      "textDocument": {
+        "synchronization": {
+          "dynamicRegistration": true,
+          "willSave": true,
+          "willSaveWaitUntil": true,
+          "didSave": true
+        }
+      }
+    }
+  }
+}
diff --git a/cli/tests/lsp/initialized_notification.json b/cli/tests/lsp/initialized_notification.json
new file mode 100644
index 000000000..972f8abc8
--- /dev/null
+++ b/cli/tests/lsp/initialized_notification.json
@@ -0,0 +1,5 @@
+{
+  "jsonrpc": "2.0",
+  "method": "initialized",
+  "params": {}
+}
diff --git a/cli/tests/lsp/shutdown_request.json b/cli/tests/lsp/shutdown_request.json
new file mode 100644
index 000000000..fd4d78460
--- /dev/null
+++ b/cli/tests/lsp/shutdown_request.json
@@ -0,0 +1,6 @@
+{
+  "jsonrpc": "2.0",
+  "id": 3,
+  "method": "shutdown",
+  "params": null
+}
diff --git a/cli/tests/lsp_tests.rs b/cli/tests/lsp_tests.rs
new file mode 100644
index 000000000..7de655ac8
--- /dev/null
+++ b/cli/tests/lsp_tests.rs
@@ -0,0 +1,88 @@
+// Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
+
+///!
+///! Integration test for the Deno Language Server (`deno lsp`)
+///!
+use std::fs;
+use std::io::Read;
+use std::io::Write;
+use std::process::Stdio;
+
+struct LspIntegrationTest {
+  pub fixtures: Vec<&'static str>,
+}
+
+impl LspIntegrationTest {
+  pub fn run(&self) -> (String, String) {
+    let root_path = test_util::root_path();
+    let deno_exe = test_util::deno_exe_path();
+    let tests_dir = root_path.join("cli/tests/lsp");
+    println!("tests_dir: {:?} deno_exe: {:?}", tests_dir, deno_exe);
+    let mut command = test_util::deno_cmd();
+    command
+      .arg("lsp")
+      .stdin(Stdio::piped())
+      .stdout(Stdio::piped())
+      .stderr(Stdio::piped());
+
+    let process = command.spawn().expect("failed to execute deno");
+
+    for fixture in &self.fixtures {
+      let mut stdin = process.stdin.as_ref().unwrap();
+      let fixture_path = tests_dir.join(fixture);
+      let content =
+        fs::read_to_string(&fixture_path).expect("could not read fixture");
+      let content_length = content.chars().count();
+      write!(
+        stdin,
+        "Content-Length: {}\r\n\r\n{}",
+        content_length, content
+      )
+      .unwrap();
+    }
+
+    let mut so = String::new();
+    process.stdout.unwrap().read_to_string(&mut so).unwrap();
+
+    let mut se = String::new();
+    process.stderr.unwrap().read_to_string(&mut se).unwrap();
+
+    (so, se)
+  }
+}
+
+#[test]
+fn test_lsp_startup_shutdown() {
+  let test = LspIntegrationTest {
+    fixtures: vec![
+      "initialize_request.json",
+      "initialized_notification.json",
+      "shutdown_request.json",
+      "exit_notification.json",
+    ],
+  };
+  let (response, out) = test.run();
+  assert!(response.contains("deno-language-server"));
+  assert!(out.contains("Connected to \"test-harness\" 1.0.0"));
+}
+
+#[test]
+fn test_lsp_hover() {
+  // a straight forward integration tests starts up the lsp, opens a document
+  // which logs `Deno.args` to the console, and hovers over the `args` property
+  // to get the intellisense about it, which is a total end-to-end test that
+  // includes sending information in and out of the TypeScript compiler.
+  let test = LspIntegrationTest {
+    fixtures: vec![
+      "initialize_request.json",
+      "initialized_notification.json",
+      "did_open_notification.json",
+      "hover_request.json",
+      "shutdown_request.json",
+      "exit_notification.json",
+    ],
+  };
+  let (response, out) = test.run();
+  assert!(response.contains("const Deno.args: string[]"));
+  assert!(out.contains("Connected to \"test-harness\" 1.0.0"));
+}
diff --git a/cli/tests/type_directives_01.ts.out b/cli/tests/type_directives_01.ts.out
index 8d285d3a8..77ed3ae26 100644
--- a/cli/tests/type_directives_01.ts.out
+++ b/cli/tests/type_directives_01.ts.out
@@ -1,3 +1,3 @@
 [WILDCARD]
-DEBUG TS - "host.getSourceFile(\"http://127.0.0.1:4545/xTypeScriptTypes.d.ts\", Latest)"
+DEBUG TS - host.getSourceFile("http://127.0.0.1:4545/xTypeScriptTypes.d.ts", Latest)
 [WILDCARD]
\ No newline at end of file
diff --git a/cli/tests/type_directives_02.ts.out b/cli/tests/type_directives_02.ts.out
index aea1d4fd0..7949dfab5 100644
--- a/cli/tests/type_directives_02.ts.out
+++ b/cli/tests/type_directives_02.ts.out
@@ -1,3 +1,3 @@
 [WILDCARD]
-DEBUG TS - "host.getSourceFile(\"file:///[WILDCARD]cli/tests/subdir/type_reference.d.ts\", Latest)"
+DEBUG TS - host.getSourceFile("file:///[WILDCARD]cli/tests/subdir/type_reference.d.ts", Latest)
 [WILDCARD]
\ No newline at end of file
diff --git a/cli/tools/lint.rs b/cli/tools/lint.rs
index c40dcfd54..dc9a51a89 100644
--- a/cli/tools/lint.rs
+++ b/cli/tools/lint.rs
@@ -122,7 +122,7 @@ pub fn print_rules_list(json: bool) {
   }
 }
 
-fn create_linter(syntax: Syntax, rules: Vec<Box<dyn LintRule>>) -> Linter {
+pub fn create_linter(syntax: Syntax, rules: Vec<Box<dyn LintRule>>) -> Linter {
   LinterBuilder::default()
     .ignore_file_directive("deno-lint-ignore-file")
     .ignore_diagnostic_directive("deno-lint-ignore")
diff --git a/cli/tsc.rs b/cli/tsc.rs
index 36668f6f7..69373b2fa 100644
--- a/cli/tsc.rs
+++ b/cli/tsc.rs
@@ -284,12 +284,12 @@ fn load(state: &mut State, args: Value) -> Result<Value, AnyError> {
 
 #[derive(Debug, Deserialize)]
 #[serde(rename_all = "camelCase")]
-struct ResolveArgs {
+pub struct ResolveArgs {
   /// The base specifier that the supplied specifier strings should be resolved
   /// relative to.
-  base: String,
+  pub base: String,
   /// A list of specifiers that should be resolved.
-  specifiers: Vec<String>,
+  pub specifiers: Vec<String>,
 }
 
 fn resolve(state: &mut State, args: Value) -> Result<Value, AnyError> {
diff --git a/cli/tsc/99_main_compiler.js b/cli/tsc/99_main_compiler.js
index bb8458c93..f379d6bae 100644
--- a/cli/tsc/99_main_compiler.js
+++ b/cli/tsc/99_main_compiler.js
@@ -1,5 +1,7 @@
 // Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
 
+// @ts-check
+/// <reference path="./compiler.d.ts" />
 // deno-lint-ignore-file no-undef
 
 // This module is the entry point for "compiler" isolate, ie. the one
@@ -11,6 +13,7 @@
 delete Object.prototype.__proto__;
 
 ((window) => {
+  /** @type {DenoCore} */
   const core = window.Deno.core;
 
   let logDebug = false;
@@ -25,7 +28,9 @@ delete Object.prototype.__proto__;
 
   function debug(...args) {
     if (logDebug) {
-      const stringifiedArgs = args.map((arg) => JSON.stringify(arg)).join(" ");
+      const stringifiedArgs = args.map((arg) =>
+        typeof arg === "string" ? arg : JSON.stringify(arg)
+      ).join(" ");
       core.print(`DEBUG ${logSource} - ${stringifiedArgs}\n`);
     }
   }
@@ -86,6 +91,7 @@ delete Object.prototype.__proto__;
   /** @param {ts.Diagnostic[]} diagnostics */
   function fromTypeScriptDiagnostic(diagnostics) {
     return diagnostics.map(({ relatedInformation: ri, source, ...diag }) => {
+      /** @type {any} */
       const value = fromRelatedInformation(diag);
       value.relatedInformation = ri
         ? ri.map(fromRelatedInformation)
@@ -106,7 +112,7 @@ delete Object.prototype.__proto__;
    * Deno, as they provide misleading or incorrect information. */
   const IGNORED_DIAGNOSTICS = [
     // TS1208: All files must be modules when the '--isolatedModules' flag is
-    // provided.  We can ignore because we guarantuee that all files are
+    // provided.  We can ignore because we guarantee that all files are
     // modules.
     1208,
     // TS1375: 'await' expressions are only allowed at the top level of a file
@@ -148,10 +154,72 @@ delete Object.prototype.__proto__;
     target: ts.ScriptTarget.ESNext,
   };
 
+  class ScriptSnapshot {
+    /** @type {string} */
+    specifier;
+    /** @type {string} */
+    version;
+    /**
+     * @param {string} specifier
+     * @param {string} version 
+     */
+    constructor(specifier, version) {
+      this.specifier = specifier;
+      this.version = version;
+    }
+    /**
+     * @param {number} start 
+     * @param {number} end 
+     * @returns {string}
+     */
+    getText(start, end) {
+      const { specifier, version } = this;
+      debug(
+        `snapshot.getText(${start}, ${end}) specifier: ${specifier} version: ${version}`,
+      );
+      return core.jsonOpSync("op_get_text", { specifier, version, start, end });
+    }
+    /**
+     * @returns {number}
+     */
+    getLength() {
+      const { specifier, version } = this;
+      debug(`snapshot.getLength() specifier: ${specifier} version: ${version}`);
+      return core.jsonOpSync("op_get_length", { specifier, version });
+    }
+    /**
+     * @param {ScriptSnapshot} oldSnapshot
+     * @returns {ts.TextChangeRange | undefined}
+     */
+    getChangeRange(oldSnapshot) {
+      const { specifier, version } = this;
+      const { version: oldVersion } = oldSnapshot;
+      const oldLength = oldSnapshot.getLength();
+      debug(
+        `snapshot.getLength() specifier: ${specifier} oldVersion: ${oldVersion} version: ${version}`,
+      );
+      return core.jsonOpSync(
+        "op_get_change_range",
+        { specifier, oldLength, oldVersion, version },
+      );
+    }
+    dispose() {
+      const { specifier, version } = this;
+      debug(`snapshot.dispose() specifier: ${specifier} version: ${version}`);
+      core.jsonOpSync("op_dispose", { specifier, version });
+    }
+  }
+
+  /** @type {ts.CompilerOptions} */
+  let compilationSettings = {};
+
+  /** @type {ts.LanguageService} */
+  let languageService;
+
   /** An object literal of the incremental compiler host, which provides the
    * specific "bindings" to the Deno environment that tsc needs to work.
    *
-   * @type {ts.CompilerHost} */
+   * @type {ts.CompilerHost & ts.LanguageServiceHost} */
   const host = {
     fileExists(fileName) {
       debug(`host.fileExists("${fileName}")`);
@@ -231,21 +299,73 @@ delete Object.prototype.__proto__;
       debug(`host.resolveModuleNames()`);
       debug(`  base: ${base}`);
       debug(`  specifiers: ${specifiers.join(", ")}`);
-      /** @type {Array<[string, ts.Extension]>} */
+      /** @type {Array<[string, ts.Extension] | undefined>} */
       const resolved = core.jsonOpSync("op_resolve", {
         specifiers,
         base,
       });
-      const r = resolved.map(([resolvedFileName, extension]) => ({
-        resolvedFileName,
-        extension,
-        isExternalLibraryImport: false,
-      }));
-      return r;
+      if (resolved) {
+        const result = resolved.map((item) => {
+          if (item) {
+            const [resolvedFileName, extension] = item;
+            return {
+              resolvedFileName,
+              extension,
+              isExternalLibraryImport: false,
+            };
+          }
+          return undefined;
+        });
+        result.length = specifiers.length;
+        return result;
+      } else {
+        return new Array(specifiers.length);
+      }
     },
     createHash(data) {
       return core.jsonOpSync("op_create_hash", { data }).hash;
     },
+
+    // LanguageServiceHost
+    getCompilationSettings() {
+      debug("host.getCompilationSettings()");
+      return compilationSettings;
+    },
+    getScriptFileNames() {
+      debug("host.getScriptFileNames()");
+      return core.jsonOpSync("op_script_names", undefined);
+    },
+    getScriptVersion(specifier) {
+      debug(`host.getScriptVersion("${specifier}")`);
+      const sourceFile = sourceFileCache.get(specifier);
+      if (sourceFile) {
+        return sourceFile.version ?? "1";
+      }
+      return core.jsonOpSync("op_script_version", { specifier });
+    },
+    getScriptSnapshot(specifier) {
+      debug(`host.getScriptSnapshot("${specifier}")`);
+      const sourceFile = sourceFileCache.get(specifier);
+      if (sourceFile) {
+        return {
+          getText(start, end) {
+            return sourceFile.text.substring(start, end);
+          },
+          getLength() {
+            return sourceFile.text.length;
+          },
+          getChangeRange() {
+            return undefined;
+          },
+        };
+      }
+      /** @type {string | undefined} */
+      const version = core.jsonOpSync("op_script_version", { specifier });
+      if (version != null) {
+        return new ScriptSnapshot(specifier, version);
+      }
+      return undefined;
+    },
   };
 
   /** @type {Array<[string, number]>} */
@@ -254,10 +374,13 @@ delete Object.prototype.__proto__;
 
   function performanceStart() {
     stats.length = 0;
-    statsStart = new Date();
+    statsStart = Date.now();
     ts.performance.enable();
   }
 
+  /**
+   * @param {{ program: ts.Program | ts.EmitAndSemanticDiagnosticsBuilderProgram, fileCount?: number }} options 
+   */
   function performanceProgram({ program, fileCount }) {
     if (program) {
       if ("getProgram" in program) {
@@ -286,7 +409,7 @@ delete Object.prototype.__proto__;
   }
 
   function performanceEnd() {
-    const duration = new Date() - statsStart;
+    const duration = Date.now() - statsStart;
     stats.push(["Compile time", duration]);
     return stats;
   }
@@ -308,7 +431,7 @@ delete Object.prototype.__proto__;
     debug(config);
 
     const { options, errors: configFileParsingDiagnostics } = ts
-      .convertCompilerOptionsFromJson(config, "", "tsconfig.json");
+      .convertCompilerOptionsFromJson(config, "");
     // The `allowNonTsExtensions` is a "hidden" compiler option used in VSCode
     // which is not allowed to be passed in JSON, we need it to allow special
     // URLs which Deno supports. So we need to either ignore the diagnostic, or
@@ -340,6 +463,106 @@ delete Object.prototype.__proto__;
     debug("<<< exec stop");
   }
 
+  /**
+   * @param {number} id 
+   * @param {any} data 
+   */
+  function respond(id, data = null) {
+    core.jsonOpSync("op_respond", { id, data });
+  }
+
+  /**
+   * @param {LanguageServerRequest} request 
+   */
+  function serverRequest({ id, ...request }) {
+    debug(`serverRequest()`, { id, ...request });
+    switch (request.method) {
+      case "configure": {
+        const { options, errors } = ts
+          .convertCompilerOptionsFromJson(request.compilerOptions, "");
+        Object.assign(options, { allowNonTsExtensions: true });
+        if (errors.length) {
+          debug(ts.formatDiagnostics(errors, host));
+        }
+        compilationSettings = options;
+        return respond(id, true);
+      }
+      case "getSemanticDiagnostics": {
+        const diagnostics = languageService.getSemanticDiagnostics(
+          request.specifier,
+        ).filter(({ code }) => !IGNORED_DIAGNOSTICS.includes(code));
+        return respond(id, fromTypeScriptDiagnostic(diagnostics));
+      }
+      case "getSuggestionDiagnostics": {
+        const diagnostics = languageService.getSuggestionDiagnostics(
+          request.specifier,
+        ).filter(({ code }) => !IGNORED_DIAGNOSTICS.includes(code));
+        return respond(id, fromTypeScriptDiagnostic(diagnostics));
+      }
+      case "getSyntacticDiagnostics": {
+        const diagnostics = languageService.getSyntacticDiagnostics(
+          request.specifier,
+        ).filter(({ code }) => !IGNORED_DIAGNOSTICS.includes(code));
+        return respond(id, fromTypeScriptDiagnostic(diagnostics));
+      }
+      case "getQuickInfo": {
+        return respond(
+          id,
+          languageService.getQuickInfoAtPosition(
+            request.specifier,
+            request.position,
+          ),
+        );
+      }
+      case "getDocumentHighlights": {
+        return respond(
+          id,
+          languageService.getDocumentHighlights(
+            request.specifier,
+            request.position,
+            request.filesToSearch,
+          ),
+        );
+      }
+      case "getReferences": {
+        return respond(
+          id,
+          languageService.getReferencesAtPosition(
+            request.specifier,
+            request.position,
+          ),
+        );
+      }
+      case "getDefinition": {
+        return respond(
+          id,
+          languageService.getDefinitionAndBoundSpan(
+            request.specifier,
+            request.position,
+          ),
+        );
+      }
+      default:
+        throw new TypeError(
+          // @ts-ignore exhausted case statement sets type to never
+          `Invalid request method for request: "${request.method}" (${id})`,
+        );
+    }
+  }
+
+  /** @param {{ debug: boolean; }} init */
+  function serverInit({ debug: debugFlag }) {
+    if (hasStarted) {
+      throw new Error("The language server has already been initialized.");
+    }
+    hasStarted = true;
+    languageService = ts.createLanguageService(host);
+    core.ops();
+    core.registerErrorClass("Error", Error);
+    setLogDebug(debugFlag, "TSLS");
+    debug("serverInit()");
+  }
+
   let hasStarted = false;
 
   /** Startup the runtime environment, setting various flags.
@@ -391,4 +614,9 @@ delete Object.prototype.__proto__;
   // checking TypeScript.
   globalThis.startup = startup;
   globalThis.exec = exec;
+
+  // exposes the functions that are called when the compiler is used as a
+  // language service.
+  globalThis.serverInit = serverInit;
+  globalThis.serverRequest = serverRequest;
 })(this);
diff --git a/cli/tsc/compiler.d.ts b/cli/tsc/compiler.d.ts
new file mode 100644
index 000000000..1a899c291
--- /dev/null
+++ b/cli/tsc/compiler.d.ts
@@ -0,0 +1,103 @@
+// Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
+
+// Contains types that can be used to validate and check `99_main_compiler.js`
+
+import * as _ts from "../dts/typescript";
+
+declare global {
+  // deno-lint-ignore no-namespace
+  namespace ts {
+    var libs: string[];
+    var libMap: Map<string, string>;
+
+    interface SourceFile {
+      version?: string;
+    }
+
+    interface Performance {
+      enable(): void;
+      getDuration(value: string): number;
+    }
+
+    var performance: Performance;
+  }
+
+  // deno-lint-ignore no-namespace
+  namespace ts {
+    export = _ts;
+  }
+
+  interface Object {
+    // deno-lint-ignore no-explicit-any
+    __proto__: any;
+  }
+
+  interface DenoCore {
+    // deno-lint-ignore no-explicit-any
+    jsonOpSync<T>(name: string, params: T): any;
+    ops(): void;
+    print(msg: string): void;
+    registerErrorClass(name: string, Ctor: typeof Error): void;
+  }
+
+  type LanguageServerRequest =
+    | ConfigureRequest
+    | GetSyntacticDiagnosticsRequest
+    | GetSemanticDiagnosticsRequest
+    | GetSuggestionDiagnosticsRequest
+    | GetQuickInfoRequest
+    | GetDocumentHighlightsRequest
+    | GetReferencesRequest
+    | GetDefinitionRequest;
+
+  interface BaseLanguageServerRequest {
+    id: number;
+    method: string;
+  }
+
+  interface ConfigureRequest extends BaseLanguageServerRequest {
+    method: "configure";
+    // deno-lint-ignore no-explicit-any
+    compilerOptions: Record<string, any>;
+  }
+
+  interface GetSyntacticDiagnosticsRequest extends BaseLanguageServerRequest {
+    method: "getSyntacticDiagnostics";
+    specifier: string;
+  }
+
+  interface GetSemanticDiagnosticsRequest extends BaseLanguageServerRequest {
+    method: "getSemanticDiagnostics";
+    specifier: string;
+  }
+
+  interface GetSuggestionDiagnosticsRequest extends BaseLanguageServerRequest {
+    method: "getSuggestionDiagnostics";
+    specifier: string;
+  }
+
+  interface GetQuickInfoRequest extends BaseLanguageServerRequest {
+    method: "getQuickInfo";
+    specifier: string;
+    position: number;
+  }
+
+  interface GetDocumentHighlightsRequest extends BaseLanguageServerRequest {
+    method: "getDocumentHighlights";
+    specifier: string;
+    position: number;
+    filesToSearch: string[];
+  }
+
+  interface GetReferencesRequest extends BaseLanguageServerRequest {
+    method: "getReferences";
+    specifier: string;
+    position: number;
+  }
+
+  interface GetDefinitionRequest extends BaseLanguageServerRequest {
+    method: "getDefinition";
+    specifier: string;
+    position: number;
+  }
+}
diff --git a/cli/tsc_config.rs b/cli/tsc_config.rs
index 773d2afb0..16661c768 100644
--- a/cli/tsc_config.rs
+++ b/cli/tsc_config.rs
@@ -52,7 +52,7 @@ impl fmt::Display for IgnoredCompilerOptions {
 /// A static slice of all the compiler options that should be ignored that
 /// either have no effect on the compilation or would cause the emit to not work
 /// in Deno.
-const IGNORED_COMPILER_OPTIONS: &[&str] = &[
+pub const IGNORED_COMPILER_OPTIONS: &[&str] = &[
   "allowSyntheticDefaultImports",
   "allowUmdGlobalAccess",
   "baseUrl",
@@ -83,7 +83,7 @@ const IGNORED_COMPILER_OPTIONS: &[&str] = &[
   "useDefineForClassFields",
 ];
 
-const IGNORED_RUNTIME_COMPILER_OPTIONS: &[&str] = &[
+pub const IGNORED_RUNTIME_COMPILER_OPTIONS: &[&str] = &[
   "assumeChangesOnlyAffectDirectDependencies",
   "build",
   "charset",