refactor: cleanup compiler pipeline (#2686)

* remove fetch_source_file_and_maybe_compile_async and 
  replace it with State.fetch_compiled_module

* remove SourceFile.js_source()

* introduce CompiledModule which is basically the same as
  deno::SourceInfo and represents arbitrary file that has been 
  compiled to JS module

* introduce //cli/compilers module containing all compilers

* introduce JsCompiler which is a no-op compiler
  - output is the same as input, no compilation takes place
  - it is used for MediaType::JavaScript and MediaType::Unknown

* introduce JsonCompiler that wraps JSON in default export

* support JS-to-JS compilation using checkJs

14 files changed, 353 insertions, 199 deletions

diff --git a/cli/compilers/js.rs b/cli/compilers/js.rs
new file mode 100644
index 000000000..56c9b672e
--- /dev/null
+++ b/cli/compilers/js.rs
@@ -0,0 +1,25 @@
+// Copyright 2018-2019 the Deno authors. All rights reserved. MIT license.
+use crate::compilers::CompiledModule;
+use crate::compilers::CompiledModuleFuture;
+use crate::file_fetcher::SourceFile;
+use crate::state::ThreadSafeState;
+use std::str;
+
+pub struct JsCompiler {}
+
+impl JsCompiler {
+  pub fn compile_async(
+    self: &Self,
+    _state: ThreadSafeState,
+    source_file: &SourceFile,
+  ) -> Box<CompiledModuleFuture> {
+    let module = CompiledModule {
+      code: str::from_utf8(&source_file.source_code)
+        .unwrap()
+        .to_string(),
+      name: source_file.url.to_string(),
+    };
+
+    Box::new(futures::future::ok(module))
+  }
+}
diff --git a/cli/compilers/json.rs b/cli/compilers/json.rs
new file mode 100644
index 000000000..57e44d354
--- /dev/null
+++ b/cli/compilers/json.rs
@@ -0,0 +1,26 @@
+// Copyright 2018-2019 the Deno authors. All rights reserved. MIT license.
+use crate::compilers::CompiledModule;
+use crate::compilers::CompiledModuleFuture;
+use crate::file_fetcher::SourceFile;
+use crate::state::ThreadSafeState;
+use std::str;
+
+pub struct JsonCompiler {}
+
+impl JsonCompiler {
+  pub fn compile_async(
+    self: &Self,
+    _state: ThreadSafeState,
+    source_file: &SourceFile,
+  ) -> Box<CompiledModuleFuture> {
+    let module = CompiledModule {
+      code: format!(
+        "export default {};",
+        str::from_utf8(&source_file.source_code).unwrap()
+      ),
+      name: source_file.url.to_string(),
+    };
+
+    Box::new(futures::future::ok(module))
+  }
+}
diff --git a/cli/compilers/mod.rs b/cli/compilers/mod.rs
new file mode 100644
index 000000000..fdc18d2bc
--- /dev/null
+++ b/cli/compilers/mod.rs
@@ -0,0 +1,20 @@
+// Copyright 2018-2019 the Deno authors. All rights reserved. MIT license.
+use deno::ErrBox;
+use futures::Future;
+
+mod js;
+mod json;
+mod ts;
+
+pub use js::JsCompiler;
+pub use json::JsonCompiler;
+pub use ts::TsCompiler;
+
+#[derive(Debug, Clone)]
+pub struct CompiledModule {
+  pub code: String,
+  pub name: String,
+}
+
+pub type CompiledModuleFuture =
+  dyn Future<Item = CompiledModule, Error = ErrBox> + Send;
diff --git a/cli/compiler.rs b/cli/compilers/ts.rs
index f90337d02..bbfe33461 100644
--- a/cli/compiler.rs
+++ b/cli/compilers/ts.rs
@@ -1,9 +1,13 @@
 // Copyright 2018-2019 the Deno authors. All rights reserved. MIT license.
+use crate::compilers::CompiledModule;
+use crate::compilers::CompiledModuleFuture;
+use crate::deno_error::DenoError;
 use crate::diagnostics::Diagnostic;
 use crate::disk_cache::DiskCache;
 use crate::file_fetcher::SourceFile;
 use crate::file_fetcher::SourceFileFetcher;
 use crate::msg;
+use crate::msg::ErrorKind;
 use crate::resources;
 use crate::source_maps::SourceMapGetter;
 use crate::startup_data;
@@ -13,7 +17,6 @@ use crate::worker::Worker;
 use deno::Buf;
 use deno::ErrBox;
 use deno::ModuleSpecifier;
-use futures::future::Either;
 use futures::Future;
 use futures::Stream;
 use ring;
@@ -26,10 +29,78 @@ use std::sync::atomic::Ordering;
 use std::sync::Mutex;
 use url::Url;
 
-/// Optional tuple which represents the state of the compiler
-/// configuration where the first is canonical name for the configuration file
-/// and a vector of the bytes of the contents of the configuration file.
-type CompilerConfig = Option<(PathBuf, Vec<u8>)>;
+/// Struct which represents the state of the compiler
+/// configuration where the first is canonical name for the configuration file,
+/// second is a vector of the bytes of the contents of the configuration file,
+/// third is bytes of the hash of contents.
+#[derive(Clone)]
+pub struct CompilerConfig {
+  pub path: Option<PathBuf>,
+  pub content: Option<Vec<u8>>,
+  pub hash: Vec<u8>,
+}
+
+impl CompilerConfig {
+  /// Take the passed flag and resolve the file name relative to the cwd.
+  pub fn load(config_path: Option<String>) -> Result<Self, ErrBox> {
+    let config_file = match &config_path {
+      Some(config_file_name) => {
+        debug!("Compiler config file: {}", config_file_name);
+        let cwd = std::env::current_dir().unwrap();
+        Some(cwd.join(config_file_name))
+      }
+      _ => None,
+    };
+
+    // Convert the PathBuf to a canonicalized string.  This is needed by the
+    // compiler to properly deal with the configuration.
+    let config_path = match &config_file {
+      Some(config_file) => Some(config_file.canonicalize().unwrap().to_owned()),
+      _ => None,
+    };
+
+    // Load the contents of the configuration file
+    let config = match &config_file {
+      Some(config_file) => {
+        debug!("Attempt to load config: {}", config_file.to_str().unwrap());
+        let config = fs::read(&config_file)?;
+        Some(config)
+      }
+      _ => None,
+    };
+
+    let config_hash = match &config {
+      Some(bytes) => bytes.clone(),
+      _ => b"".to_vec(),
+    };
+
+    let ts_config = Self {
+      path: config_path,
+      content: config,
+      hash: config_hash,
+    };
+
+    Ok(ts_config)
+  }
+
+  pub fn json(self: &Self) -> Result<serde_json::Value, ErrBox> {
+    if self.content.is_none() {
+      return Ok(serde_json::Value::Null);
+    }
+
+    let bytes = self.content.clone().unwrap();
+    let json_string = std::str::from_utf8(&bytes)?;
+    match serde_json::from_str(&json_string) {
+      Ok(json_map) => Ok(json_map),
+      Err(_) => Err(
+        DenoError::new(
+          ErrorKind::InvalidInput,
+          "Compiler config is not a valid JSON".to_string(),
+        ).into(),
+      ),
+    }
+  }
+}
 
 /// Information associated with compiled file in cache.
 /// Includes source code path and state hash.
@@ -80,19 +151,19 @@ fn req(
   compiler_config: CompilerConfig,
   bundle: Option<String>,
 ) -> Buf {
-  let j = if let Some((config_path, config_data)) = compiler_config {
-    json!({
-      "rootNames": root_names,
-      "bundle": bundle,
-      "configPath": config_path,
-      "config": str::from_utf8(&config_data).unwrap(),
-    })
-  } else {
-    json!({
-      "rootNames": root_names,
-      "bundle": bundle,
-    })
+  let j = match (compiler_config.path, compiler_config.content) {
+    (Some(config_path), Some(config_data)) => json!({
+        "rootNames": root_names,
+        "bundle": bundle,
+        "configPath": config_path,
+        "config": str::from_utf8(&config_data).unwrap(),
+      }),
+    _ => json!({
+        "rootNames": root_names,
+        "bundle": bundle,
+      }),
   };
+
   j.to_string().into_boxed_str().into_boxed_bytes()
 }
 
@@ -120,48 +191,9 @@ pub fn source_code_version_hash(
   gen_hash(vec![source_code, version.as_bytes(), config_hash])
 }
 
-fn load_config_file(
-  config_path: Option<String>,
-) -> (Option<PathBuf>, Option<Vec<u8>>) {
-  // take the passed flag and resolve the file name relative to the cwd
-  let config_file = match &config_path {
-    Some(config_file_name) => {
-      debug!("Compiler config file: {}", config_file_name);
-      let cwd = std::env::current_dir().unwrap();
-      Some(cwd.join(config_file_name))
-    }
-    _ => None,
-  };
-
-  // Convert the PathBuf to a canonicalized string.  This is needed by the
-  // compiler to properly deal with the configuration.
-  let config_path = match &config_file {
-    Some(config_file) => Some(config_file.canonicalize().unwrap().to_owned()),
-    _ => None,
-  };
-
-  // Load the contents of the configuration file
-  let config = match &config_file {
-    Some(config_file) => {
-      debug!("Attempt to load config: {}", config_file.to_str().unwrap());
-      match fs::read(&config_file) {
-        Ok(config_data) => Some(config_data.to_owned()),
-        _ => panic!(
-          "Error retrieving compiler config file at \"{}\"",
-          config_file.to_str().unwrap()
-        ),
-      }
-    }
-    _ => None,
-  };
-
-  (config_path, config)
-}
-
 pub struct TsCompiler {
   pub file_fetcher: SourceFileFetcher,
   pub config: CompilerConfig,
-  pub config_hash: Vec<u8>,
   pub disk_cache: DiskCache,
   /// Set of all URLs that have been compiled. This prevents double
   /// compilation of module.
@@ -169,6 +201,8 @@ pub struct TsCompiler {
   /// This setting is controlled by `--reload` flag. Unless the flag
   /// is provided disk cache is used.
   pub use_disk_cache: bool,
+  /// This setting is controlled by `compilerOptions.checkJs`
+  pub compile_js: bool,
 }
 
 impl TsCompiler {
@@ -177,25 +211,30 @@ impl TsCompiler {
     disk_cache: DiskCache,
     use_disk_cache: bool,
     config_path: Option<String>,
-  ) -> Self {
-    let compiler_config = match load_config_file(config_path) {
-      (Some(config_path), Some(config)) => Some((config_path, config.to_vec())),
-      _ => None,
-    };
-
-    let config_bytes = match &compiler_config {
-      Some((_, config)) => config.clone(),
-      _ => b"".to_vec(),
+  ) -> Result<Self, ErrBox> {
+    let config = CompilerConfig::load(config_path)?;
+
+    // If `checkJs` is set to true in `compilerOptions` then we're gonna be compiling
+    // JavaScript files as well
+    let config_json = config.json()?;
+    let compile_js = match &config_json.get("compilerOptions") {
+      Some(serde_json::Value::Object(m)) => match m.get("checkJs") {
+        Some(serde_json::Value::Bool(bool_)) => *bool_,
+        _ => false,
+      },
+      _ => false,
     };
 
-    Self {
+    let compiler = Self {
       file_fetcher,
       disk_cache,
-      config: compiler_config,
-      config_hash: config_bytes,
+      config,
       compiled: Mutex::new(HashSet::new()),
       use_disk_cache,
-    }
+      compile_js,
+    };
+
+    Ok(compiler)
   }
 
   /// Create a new V8 worker with snapshot of TS compiler and setup compiler's runtime.
@@ -290,22 +329,12 @@ impl TsCompiler {
     self: &Self,
     state: ThreadSafeState,
     source_file: &SourceFile,
-  ) -> impl Future<Item = SourceFile, Error = ErrBox> {
-    // TODO: maybe fetching of original SourceFile should be done here?
-
-    if source_file.media_type != msg::MediaType::TypeScript {
-      return Either::A(futures::future::ok(source_file.clone()));
-    }
-
+  ) -> Box<CompiledModuleFuture> {
     if self.has_compiled(&source_file.url) {
-      match self.get_compiled_source_file(&source_file) {
-        Ok(compiled_module) => {
-          return Either::A(futures::future::ok(compiled_module));
-        }
-        Err(err) => {
-          return Either::A(futures::future::err(err));
-        }
-      }
+      return match self.get_compiled_module(&source_file.url) {
+        Ok(compiled) => Box::new(futures::future::ok(compiled)),
+        Err(err) => Box::new(futures::future::err(err)),
+      };
     }
 
     if self.use_disk_cache {
@@ -317,20 +346,16 @@ impl TsCompiler {
         let version_hash_to_validate = source_code_version_hash(
           &source_file.source_code,
           version::DENO,
-          &self.config_hash,
+          &self.config.hash,
         );
 
         if metadata.version_hash == version_hash_to_validate {
           debug!("load_cache metadata version hash match");
           if let Ok(compiled_module) =
-            self.get_compiled_source_file(&source_file)
+            self.get_compiled_module(&source_file.url)
           {
-            debug!(
-              "found cached compiled module: {:?}",
-              compiled_module.clone().filename
-            );
-            // TODO: store in in-process cache for subsequent access
-            return Either::A(futures::future::ok(compiled_module));
+            self.mark_compiled(&source_file.url);
+            return Box::new(futures::future::ok(compiled_module));
           }
         }
       }
@@ -388,19 +413,18 @@ impl TsCompiler {
       }).and_then(move |_| {
         // if we are this far it means compilation was successful and we can
         // load compiled filed from disk
-        // TODO: can this be somehow called using `self.`?
         state_
           .ts_compiler
-          .get_compiled_source_file(&source_file_)
+          .get_compiled_module(&source_file_.url)
           .map_err(|e| {
             // TODO: this situation shouldn't happen
             panic!("Expected to find compiled file: {}", e)
           })
-      }).and_then(move |source_file_after_compile| {
+      }).and_then(move |compiled_module| {
         // Explicit drop to keep reference alive until future completes.
         drop(compiling_job);
 
-        Ok(source_file_after_compile)
+        Ok(compiled_module)
       }).then(move |r| {
         debug!(">>>>> compile_sync END");
         // TODO(ry) do this in worker's destructor.
@@ -408,7 +432,7 @@ impl TsCompiler {
         r
       });
 
-    Either::B(fut)
+    Box::new(fut)
   }
 
   /// Get associated `CompiledFileMetadata` for given module if it exists.
@@ -431,22 +455,38 @@ impl TsCompiler {
     None
   }
 
+  pub fn get_compiled_module(
+    self: &Self,
+    module_url: &Url,
+  ) -> Result<CompiledModule, ErrBox> {
+    let compiled_source_file = self.get_compiled_source_file(module_url)?;
+
+    let compiled_module = CompiledModule {
+      code: str::from_utf8(&compiled_source_file.source_code)
+        .unwrap()
+        .to_string(),
+      name: module_url.to_string(),
+    };
+
+    Ok(compiled_module)
+  }
+
   /// Return compiled JS file for given TS module.
   // TODO: ideally we shouldn't construct SourceFile by hand, but it should be delegated to
   // SourceFileFetcher
   pub fn get_compiled_source_file(
     self: &Self,
-    source_file: &SourceFile,
+    module_url: &Url,
   ) -> Result<SourceFile, ErrBox> {
     let cache_key = self
       .disk_cache
-      .get_cache_filename_with_extension(&source_file.url, "js");
+      .get_cache_filename_with_extension(&module_url, "js");
     let compiled_code = self.disk_cache.get(&cache_key)?;
     let compiled_code_filename = self.disk_cache.location.join(cache_key);
     debug!("compiled filename: {:?}", compiled_code_filename);
 
     let compiled_module = SourceFile {
-      url: source_file.url.clone(),
+      url: module_url.clone(),
       filename: compiled_code_filename,
       media_type: msg::MediaType::JavaScript,
       source_code: compiled_code,
@@ -481,7 +521,7 @@ impl TsCompiler {
         let version_hash = source_code_version_hash(
           &source_file.source_code,
           version::DENO,
-          &self.config_hash,
+          &self.config.hash,
         );
 
         let compiled_file_metadata = CompiledFileMetadata {
@@ -619,7 +659,7 @@ mod tests {
       self: &Self,
       state: ThreadSafeState,
       source_file: &SourceFile,
-    ) -> Result<SourceFile, ErrBox> {
+    ) -> Result<CompiledModule, ErrBox> {
       tokio_util::block_on(self.compile_async(state, source_file))
     }
   }
@@ -630,24 +670,25 @@ mod tests {
       let specifier =
         ModuleSpecifier::resolve_url_or_path("./tests/002_hello.ts").unwrap();
 
-      let mut out = SourceFile {
+      let out = SourceFile {
         url: specifier.as_url().clone(),
         filename: PathBuf::from("/tests/002_hello.ts"),
         media_type: msg::MediaType::TypeScript,
-        source_code: include_bytes!("../tests/002_hello.ts").to_vec(),
+        source_code: include_bytes!("../../tests/002_hello.ts").to_vec(),
       };
 
       let mock_state = ThreadSafeState::mock(vec![
         String::from("./deno"),
         String::from("hello.js"),
       ]);
-      out = mock_state
+      let compiled = mock_state
         .ts_compiler
         .compile_sync(mock_state.clone(), &out)
         .unwrap();
       assert!(
-        out
-          .source_code
+        compiled
+          .code
+          .as_bytes()
           .starts_with("console.log(\"Hello World\");".as_bytes())
       );
     })
diff --git a/cli/deno_dir.rs b/cli/deno_dir.rs
index c600b06fc..ac35922eb 100644
--- a/cli/deno_dir.rs
+++ b/cli/deno_dir.rs
@@ -17,8 +17,6 @@ pub struct DenoDir {
 }
 
 impl DenoDir {
-  // Must be called before using any function from this module.
-  // https://github.com/denoland/deno/blob/golang/deno_dir.go#L99-L111
   pub fn new(custom_root: Option<PathBuf>) -> std::io::Result<Self> {
     // Only setup once.
     let home_dir = dirs::home_dir().expect("Could not get home directory.");
diff --git a/cli/disk_cache.rs b/cli/disk_cache.rs
index 808cfe675..fdbe2cbd5 100644
--- a/cli/disk_cache.rs
+++ b/cli/disk_cache.rs
@@ -18,6 +18,10 @@ impl DiskCache {
     }
   }
 
+  // TODO(bartlomieju) this method is not working properly for Windows paths,
+  // Example: file:///C:/deno/js/unit_test_runner.ts
+  // would produce: C:deno\\js\\unit_test_runner.ts
+  // it should produce: file\deno\js\unit_test_runner.ts
   pub fn get_cache_filename(self: &Self, url: &Url) -> PathBuf {
     let mut out = PathBuf::new();
 
diff --git a/cli/file_fetcher.rs b/cli/file_fetcher.rs
index 656ecfff0..79d6ede00 100644
--- a/cli/file_fetcher.rs
+++ b/cli/file_fetcher.rs
@@ -39,27 +39,6 @@ pub struct SourceFile {
   pub source_code: Vec<u8>,
 }
 
-impl SourceFile {
-  // TODO(bartlomieju): this method should be implemented on new `CompiledSourceFile`
-  // trait and should be handled by "compiler pipeline"
-  pub fn js_source(&self) -> String {
-    if self.media_type == msg::MediaType::TypeScript {
-      panic!("TypeScript module has no JS source, did you forget to run it through compiler?");
-    }
-
-    // TODO: this should be done by compiler and JS module should be returned
-    if self.media_type == msg::MediaType::Json {
-      return format!(
-        "export default {};",
-        str::from_utf8(&self.source_code).unwrap()
-      );
-    }
-
-    // it's either JS or Unknown media type
-    str::from_utf8(&self.source_code).unwrap().to_string()
-  }
-}
-
 pub type SourceFileFuture =
   dyn Future<Item = SourceFile, Error = ErrBox> + Send;
 
diff --git a/cli/main.rs b/cli/main.rs
index 452cdfa65..fb34a2c76 100644
--- a/cli/main.rs
+++ b/cli/main.rs
@@ -16,7 +16,7 @@ extern crate rand;
 extern crate url;
 
 mod ansi;
-pub mod compiler;
+pub mod compilers;
 pub mod deno_dir;
 pub mod deno_error;
 pub mod diagnostics;
@@ -99,6 +99,7 @@ fn js_check(r: Result<(), ErrBox>) {
   }
 }
 
+// TODO: we might want to rethink how this method works
 pub fn print_file_info(
   worker: Worker,
   module_specifier: &ModuleSpecifier,
@@ -110,7 +111,7 @@ pub fn print_file_info(
     .file_fetcher
     .fetch_source_file_async(&module_specifier)
     .map_err(|err| println!("{}", err))
-    .and_then(move |out| {
+    .and_then(|out| {
       println!(
         "{} {}",
         ansi::bold("local:".to_string()),
@@ -125,18 +126,25 @@ pub fn print_file_info(
 
       state_
         .clone()
-        .ts_compiler
-        .compile_async(state_.clone(), &out)
+        .fetch_compiled_module(&module_specifier_)
         .map_err(|e| {
           debug!("compiler error exiting!");
           eprintln!("\n{}", e.to_string());
           std::process::exit(1);
         }).and_then(move |compiled| {
-          if out.media_type == msg::MediaType::TypeScript {
+          if out.media_type == msg::MediaType::TypeScript
+            || (out.media_type == msg::MediaType::JavaScript
+              && state_.ts_compiler.compile_js)
+          {
+            let compiled_source_file = state_
+              .ts_compiler
+              .get_compiled_source_file(&out.url)
+              .unwrap();
+
             println!(
               "{} {}",
               ansi::bold("compiled:".to_string()),
-              compiled.filename.to_str().unwrap(),
+              compiled_source_file.filename.to_str().unwrap(),
             );
           }
 
@@ -152,12 +160,8 @@ pub fn print_file_info(
             );
           }
 
-          if let Some(deps) = worker
-            .state
-            .modules
-            .lock()
-            .unwrap()
-            .deps(&compiled.url.to_string())
+          if let Some(deps) =
+            worker.state.modules.lock().unwrap().deps(&compiled.name)
           {
             println!("{}{}", ansi::bold("deps:\n".to_string()), deps.name);
             if let Some(ref depsdeps) = deps.deps {
diff --git a/cli/state.rs b/cli/state.rs
index cd7c1269d..047e2b7ed 100644
--- a/cli/state.rs
+++ b/cli/state.rs
@@ -1,11 +1,14 @@
 // Copyright 2018-2019 the Deno authors. All rights reserved. MIT license.
-use crate::compiler::TsCompiler;
+use crate::compilers::CompiledModule;
+use crate::compilers::JsCompiler;
+use crate::compilers::JsonCompiler;
+use crate::compilers::TsCompiler;
 use crate::deno_dir;
-use crate::file_fetcher::SourceFile;
 use crate::file_fetcher::SourceFileFetcher;
 use crate::flags;
 use crate::global_timer::GlobalTimer;
 use crate::import_map::ImportMap;
+use crate::msg;
 use crate::ops;
 use crate::permissions::DenoPermissions;
 use crate::progress::Progress;
@@ -26,6 +29,7 @@ use std;
 use std::collections::HashMap;
 use std::env;
 use std::ops::Deref;
+use std::str;
 use std::sync::atomic::{AtomicUsize, Ordering};
 use std::sync::Arc;
 use std::sync::Mutex;
@@ -77,6 +81,8 @@ pub struct State {
   pub seeded_rng: Option<Mutex<StdRng>>,
 
   pub file_fetcher: SourceFileFetcher,
+  pub js_compiler: JsCompiler,
+  pub json_compiler: JsonCompiler,
   pub ts_compiler: TsCompiler,
 }
 
@@ -103,28 +109,6 @@ impl ThreadSafeState {
   }
 }
 
-pub fn fetch_source_file_and_maybe_compile_async(
-  state: &ThreadSafeState,
-  module_specifier: &ModuleSpecifier,
-) -> impl Future<Item = SourceFile, Error = ErrBox> {
-  let state_ = state.clone();
-
-  state_
-    .file_fetcher
-    .fetch_source_file_async(&module_specifier)
-    .and_then(move |out| {
-      state_
-        .clone()
-        .ts_compiler
-        .compile_async(state_.clone(), &out)
-        .map_err(|e| {
-          debug!("compiler error exiting!");
-          eprintln!("\n{}", e.to_string());
-          std::process::exit(1);
-        })
-    })
-}
-
 impl Loader for ThreadSafeState {
   fn resolve(
     &self,
@@ -150,16 +134,14 @@ impl Loader for ThreadSafeState {
     module_specifier: &ModuleSpecifier,
   ) -> Box<deno::SourceCodeInfoFuture> {
     self.metrics.resolve_count.fetch_add(1, Ordering::SeqCst);
-    Box::new(
-      fetch_source_file_and_maybe_compile_async(self, module_specifier).map(
-        |source_file| deno::SourceCodeInfo {
-          // Real module name, might be different from initial specifier
-          // due to redirections.
-          code: source_file.js_source(),
-          module_name: source_file.url.to_string(),
-        },
-      ),
-    )
+    Box::new(self.fetch_compiled_module(module_specifier).map(
+      |compiled_module| deno::SourceCodeInfo {
+        // Real module name, might be different from initial specifier
+        // due to redirections.
+        code: compiled_module.code,
+        module_name: compiled_module.name,
+      },
+    ))
   }
 }
 
@@ -192,36 +174,26 @@ impl ThreadSafeState {
       dir.gen_cache.clone(),
       !flags.reload,
       flags.config_path.clone(),
-    );
+    )?;
 
     let main_module: Option<ModuleSpecifier> = if argv_rest.len() <= 1 {
       None
     } else {
       let root_specifier = argv_rest[1].clone();
-      match ModuleSpecifier::resolve_url_or_path(&root_specifier) {
-        Ok(specifier) => Some(specifier),
-        Err(e) => {
-          // TODO: handle unresolvable specifier
-          panic!("Unable to resolve root specifier: {:?}", e);
-        }
-      }
+      Some(ModuleSpecifier::resolve_url_or_path(&root_specifier)?)
     };
 
-    let mut import_map = None;
-    if let Some(file_name) = &flags.import_map_path {
-      let base_url = match &main_module {
-        Some(module_specifier) => module_specifier.clone(),
-        None => unreachable!(),
-      };
-
-      match ImportMap::load(&base_url.to_string(), file_name) {
-        Ok(map) => import_map = Some(map),
-        Err(err) => {
-          println!("{:?}", err);
-          panic!("Error parsing import map");
-        }
+    let import_map: Option<ImportMap> = match &flags.import_map_path {
+      None => None,
+      Some(file_name) => {
+        let base_url = match &main_module {
+          Some(module_specifier) => module_specifier.clone(),
+          None => unreachable!(),
+        };
+        let import_map = ImportMap::load(&base_url.to_string(), file_name)?;
+        Some(import_map)
       }
-    }
+    };
 
     let mut seeded_rng = None;
     if let Some(seed) = flags.seed {
@@ -249,11 +221,42 @@ impl ThreadSafeState {
       seeded_rng,
       file_fetcher,
       ts_compiler,
+      js_compiler: JsCompiler {},
+      json_compiler: JsonCompiler {},
     };
 
     Ok(ThreadSafeState(Arc::new(state)))
   }
 
+  pub fn fetch_compiled_module(
+    self: &Self,
+    module_specifier: &ModuleSpecifier,
+  ) -> impl Future<Item = CompiledModule, Error = ErrBox> {
+    let state_ = self.clone();
+
+    self
+      .file_fetcher
+      .fetch_source_file_async(&module_specifier)
+      .and_then(move |out| match out.media_type {
+        msg::MediaType::Unknown => {
+          state_.js_compiler.compile_async(state_.clone(), &out)
+        }
+        msg::MediaType::Json => {
+          state_.json_compiler.compile_async(state_.clone(), &out)
+        }
+        msg::MediaType::TypeScript => {
+          state_.ts_compiler.compile_async(state_.clone(), &out)
+        }
+        msg::MediaType::JavaScript => {
+          if state_.ts_compiler.compile_js {
+            state_.ts_compiler.compile_async(state_.clone(), &out)
+          } else {
+            state_.js_compiler.compile_async(state_.clone(), &out)
+          }
+        }
+      })
+  }
+
   /// Read main module from argv
   pub fn main_module(&self) -> Option<ModuleSpecifier> {
     match &self.main_module {
diff --git a/js/compiler.ts b/js/compiler.ts
index 34ac2f482..4203f753b 100644
--- a/js/compiler.ts
+++ b/js/compiler.ts
@@ -219,6 +219,8 @@ function getExtension(
 }
 
 class Host implements ts.CompilerHost {
+  extensionCache: Record<string, ts.Extension> = {};
+
   private readonly _options: ts.CompilerOptions = {
     allowJs: true,
     allowNonTsExtensions: true,
@@ -370,10 +372,16 @@ class Host implements ts.CompilerHost {
           // This flags to the compiler to not go looking to transpile functional
           // code, anything that is in `/$asset$/` is just library code
           const isExternalLibraryImport = moduleName.startsWith(ASSETS);
+          const extension = getExtension(
+            resolvedFileName,
+            SourceFile.mediaType
+          );
+          this.extensionCache[resolvedFileName] = extension;
+
           const r = {
             resolvedFileName,
             isExternalLibraryImport,
-            extension: getExtension(resolvedFileName, SourceFile.mediaType)
+            extension
           };
           return r;
         } else {
@@ -401,6 +409,21 @@ class Host implements ts.CompilerHost {
       } else {
         assert(sourceFiles != null && sourceFiles.length == 1);
         const sourceFileName = sourceFiles![0].fileName;
+        const maybeExtension = this.extensionCache[sourceFileName];
+
+        if (maybeExtension) {
+          // NOTE: If it's a `.json` file we don't want to write it to disk.
+          // JSON files are loaded and used by TS compiler to check types, but we don't want
+          // to emit them to disk because output file is the same as input file.
+          if (maybeExtension === ts.Extension.Json) {
+            return;
+          }
+
+          // NOTE: JavaScript files are only emitted to disk if `checkJs` option in on
+          if (maybeExtension === ts.Extension.Js && !this._options.checkJs) {
+            return;
+          }
+        }
 
         if (fileName.endsWith(".map")) {
           // Source Map
diff --git a/tests/038_checkjs.js b/tests/038_checkjs.js
new file mode 100644
index 000000000..628d3e376
--- /dev/null
+++ b/tests/038_checkjs.js
@@ -0,0 +1,6 @@
+// console.log intentionally misspelled to trigger a type error
+consol.log("hello world!");
+
+// the following error should be ignored and not output to the console
+// eslint-disable-next-line
+const foo = new Foo();
diff --git a/tests/038_checkjs.js.out b/tests/038_checkjs.js.out
new file mode 100644
index 000000000..deaf77211
--- /dev/null
+++ b/tests/038_checkjs.js.out
@@ -0,0 +1,15 @@
+[WILDCARD]
+error TS2552: Cannot find name 'consol'. Did you mean 'console'?
+
+[WILDCARD]tests/038_checkjs.js:2:1
+
+2 consol.log("hello world!");
+[WILDCARD]
+error TS2552: Cannot find name 'Foo'. Did you mean 'foo'?
+
+[WILDCARD]tests/038_checkjs.js:6:17
+
+6 const foo = new Foo();
+[WILDCARD]
+Found 2 errors.
+[WILDCARD]
\ No newline at end of file
diff --git a/tests/038_checkjs.test b/tests/038_checkjs.test
new file mode 100644
index 000000000..6385c9bb7
--- /dev/null
+++ b/tests/038_checkjs.test
@@ -0,0 +1,5 @@
+# checking if JS file is run through TS compiler
+args: run --reload --config tests/038_checkjs.tsconfig.json tests/038_checkjs.js
+check_stderr: true
+exit_code: 1
+output: tests/038_checkjs.js.out
diff --git a/tests/038_checkjs.tsconfig.json b/tests/038_checkjs.tsconfig.json
new file mode 100644
index 000000000..08ac60b6c
--- /dev/null
+++ b/tests/038_checkjs.tsconfig.json
@@ -0,0 +1,5 @@
+{
+  "compilerOptions": {
+    "checkJs": true
+  }
+}