Enforce media types

22 files changed, 472 insertions, 83 deletions

diff --git a/js/compiler.ts b/js/compiler.ts
index e79df83ea..c872afa14 100644
--- a/js/compiler.ts
+++ b/js/compiler.ts
@@ -1,5 +1,6 @@
 // Copyright 2018 the Deno authors. All rights reserved. MIT license.
 import * as ts from "typescript";
+import { MediaType } from "gen/msg_generated";
 import { assetSourceCode } from "./assets";
 // tslint:disable-next-line:no-circular-imports
 import * as deno from "./deno";
@@ -85,6 +86,7 @@ export class ModuleMetaData implements ts.IScriptSnapshot {
   constructor(
     public readonly moduleId: ModuleId,
     public readonly fileName: ModuleFileName,
+    public readonly mediaType: MediaType,
     public readonly sourceCode: SourceCode = "",
     public outputCode: OutputCode = ""
   ) {
@@ -107,6 +109,23 @@ export class ModuleMetaData implements ts.IScriptSnapshot {
   }
 }
 
+function getExtension(
+  fileName: ModuleFileName,
+  mediaType: MediaType
+): ts.Extension | undefined {
+  switch (mediaType) {
+    case MediaType.JavaScript:
+      return ts.Extension.Js;
+    case MediaType.TypeScript:
+      return fileName.endsWith(".d.ts") ? ts.Extension.Dts : ts.Extension.Ts;
+    case MediaType.Json:
+      return ts.Extension.Json;
+    case MediaType.Unknown:
+    default:
+      return undefined;
+  }
+}
+
 /** A singleton class that combines the TypeScript Language Service host API
  * with Deno specific APIs to provide an interface for compiling and running
  * TypeScript and JavaScript modules.
@@ -319,17 +338,6 @@ export class DenoCompiler
     return undefined;
   }
 
-  /** Resolve the `fileName` for a given `moduleSpecifier` and
-   * `containingFile`
-   */
-  private _resolveModuleName(
-    moduleSpecifier: ModuleSpecifier,
-    containingFile: ContainingFile
-  ): ModuleFileName | undefined {
-    const moduleMetaData = this.resolveModule(moduleSpecifier, containingFile);
-    return moduleMetaData ? moduleMetaData.fileName : undefined;
-  }
-
   /** Caches the resolved `fileName` in relationship to the `moduleSpecifier`
    * and `containingFile` in order to reduce calls to the privileged side
    * to retrieve the contents of a module.
@@ -479,9 +487,10 @@ export class DenoCompiler
     if (fileName && this._moduleMetaDataMap.has(fileName)) {
       return this._moduleMetaDataMap.get(fileName)!;
     }
-    let moduleId: ModuleId;
-    let sourceCode: SourceCode;
-    let outputCode: OutputCode | null;
+    let moduleId: ModuleId | undefined;
+    let mediaType = MediaType.Unknown;
+    let sourceCode: SourceCode | undefined;
+    let outputCode: OutputCode | undefined;
     if (
       moduleSpecifier.startsWith(ASSETS) ||
       containingFile.startsWith(ASSETS)
@@ -492,6 +501,7 @@ export class DenoCompiler
       moduleId = moduleSpecifier.split("/").pop()!;
       const assetName = moduleId.includes(".") ? moduleId : `${moduleId}.d.ts`;
       assert(assetName in assetSourceCode, `No such asset "${assetName}"`);
+      mediaType = MediaType.TypeScript;
       sourceCode = assetSourceCode[assetName];
       fileName = `${ASSETS}/${assetName}`;
       outputCode = "";
@@ -499,25 +509,38 @@ export class DenoCompiler
       // We query Rust with a CodeFetch message. It will load the sourceCode,
       // and if there is any outputCode cached, will return that as well.
       const fetchResponse = this._os.codeFetch(moduleSpecifier, containingFile);
-      moduleId = fetchResponse.moduleName!;
-      fileName = fetchResponse.filename!;
-      sourceCode = fetchResponse.sourceCode!;
-      outputCode = fetchResponse.outputCode!;
+      moduleId = fetchResponse.moduleName;
+      fileName = fetchResponse.filename;
+      mediaType = fetchResponse.mediaType;
+      sourceCode = fetchResponse.sourceCode;
+      outputCode = fetchResponse.outputCode;
     }
-    assert(sourceCode!.length > 0);
-    this._log("resolveModule sourceCode length:", sourceCode.length);
-    this._log("resolveModule has outputCode:", outputCode! != null);
-    this._setFileName(moduleSpecifier, containingFile, fileName);
+    assert(moduleId != null, "No module ID.");
+    assert(fileName != null, "No file name.");
+    assert(sourceCode ? sourceCode.length > 0 : false, "No source code.");
+    assert(
+      mediaType !== MediaType.Unknown,
+      `Unknown media type for: "${moduleSpecifier}" from "${containingFile}".`
+    );
+    this._log(
+      "resolveModule sourceCode length:",
+      sourceCode && sourceCode.length
+    );
+    this._log("resolveModule has outputCode:", outputCode != null);
+    this._log("resolveModule has media type:", MediaType[mediaType]);
+    // fileName is asserted above, but TypeScript does not track so not null
+    this._setFileName(moduleSpecifier, containingFile, fileName!);
     if (fileName && this._moduleMetaDataMap.has(fileName)) {
       return this._moduleMetaDataMap.get(fileName)!;
     }
     const moduleMetaData = new ModuleMetaData(
-      moduleId,
-      fileName,
+      moduleId!,
+      fileName!,
+      mediaType,
       sourceCode,
       outputCode
     );
-    this._moduleMetaDataMap.set(fileName, moduleMetaData);
+    this._moduleMetaDataMap.set(fileName!, moduleMetaData);
     return moduleMetaData;
   }
 
@@ -563,16 +586,20 @@ export class DenoCompiler
 
   getScriptKind(fileName: ModuleFileName): ts.ScriptKind {
     this._log("getScriptKind()", fileName);
-    const suffix = fileName.substr(fileName.lastIndexOf(".") + 1);
-    switch (suffix) {
-      case "ts":
-        return ts.ScriptKind.TS;
-      case "js":
-        return ts.ScriptKind.JS;
-      case "json":
-        return ts.ScriptKind.JSON;
-      default:
-        return this._options.allowJs ? ts.ScriptKind.JS : ts.ScriptKind.TS;
+    const moduleMetaData = this._getModuleMetaData(fileName);
+    if (moduleMetaData) {
+      switch (moduleMetaData.mediaType) {
+        case MediaType.TypeScript:
+          return ts.ScriptKind.TS;
+        case MediaType.JavaScript:
+          return ts.ScriptKind.JS;
+        case MediaType.Json:
+          return ts.ScriptKind.JSON;
+        default:
+          return this._options.allowJs ? ts.ScriptKind.JS : ts.ScriptKind.TS;
+      }
+    } else {
+      return this._options.allowJs ? ts.ScriptKind.JS : ts.ScriptKind.TS;
     }
   }
 
@@ -619,17 +646,17 @@ export class DenoCompiler
   resolveModuleNames(
     moduleNames: ModuleSpecifier[],
     containingFile: ContainingFile
-  ): ts.ResolvedModule[] {
+  ): Array<ts.ResolvedModuleFull | ts.ResolvedModule> {
     this._log("resolveModuleNames()", { moduleNames, containingFile });
     return moduleNames.map(name => {
-      let resolvedFileName;
+      let moduleMetaData: ModuleMetaData;
       if (name === "deno") {
         // builtin modules are part of the runtime lib
-        resolvedFileName = this._resolveModuleName(LIB_RUNTIME, ASSETS);
+        moduleMetaData = this.resolveModule(LIB_RUNTIME, ASSETS);
       } else if (name === "typescript") {
-        resolvedFileName = this._resolveModuleName("typescript.d.ts", ASSETS);
+        moduleMetaData = this.resolveModule("typescript.d.ts", ASSETS);
       } else {
-        resolvedFileName = this._resolveModuleName(name, containingFile);
+        moduleMetaData = this.resolveModule(name, containingFile);
       }
       // According to the interface we shouldn't return `undefined` but if we
       // fail to return the same length of modules to those we cannot resolve
@@ -638,12 +665,15 @@ export class DenoCompiler
       // TODO: all this does is push the problem downstream, and TypeScript
       // will complain it can't identify the type of the file and throw
       // a runtime exception, so we need to handle missing modules better
-      resolvedFileName = resolvedFileName || "";
+      const resolvedFileName = moduleMetaData.fileName || "";
       // This flags to the compiler to not go looking to transpile functional
       // code, anything that is in `/$asset$/` is just library code
       const isExternalLibraryImport = resolvedFileName.startsWith(ASSETS);
-      // TODO: we should be returning a ts.ResolveModuleFull
-      return { resolvedFileName, isExternalLibraryImport };
+      return {
+        resolvedFileName,
+        isExternalLibraryImport,
+        extension: getExtension(resolvedFileName, moduleMetaData.mediaType)
+      };
     });
   }
 
@@ -662,9 +692,7 @@ export class DenoCompiler
 
   private static _instance: DenoCompiler | undefined;
 
-  /**
-   * Returns the instance of `DenoCompiler` or creates a new instance.
-   */
+  /** Returns the instance of `DenoCompiler` or creates a new instance. */
   static instance(): DenoCompiler {
     return (
       DenoCompiler._instance || (DenoCompiler._instance = new DenoCompiler())
diff --git a/js/compiler_test.ts b/js/compiler_test.ts
index f05a96e52..7d1bdc150 100644
--- a/js/compiler_test.ts
+++ b/js/compiler_test.ts
@@ -11,6 +11,7 @@ const { DenoCompiler } = (deno as any)._compiler;
 interface ModuleInfo {
   moduleName: string | null;
   filename: string | null;
+  mediaType: MediaType | null;
   sourceCode: string | null;
   outputCode: string | null;
 }
@@ -27,15 +28,24 @@ const originals = {
   _window: (compilerInstance as any)._window
 };
 
+enum MediaType {
+  JavaScript = 0,
+  TypeScript = 1,
+  Json = 2,
+  Unknown = 3
+}
+
 function mockModuleInfo(
   moduleName: string | null,
   filename: string | null,
+  mediaType: MediaType | null,
   sourceCode: string | null,
   outputCode: string | null
 ): ModuleInfo {
   return {
     moduleName,
     filename,
+    mediaType,
     sourceCode,
     outputCode
   };
@@ -61,6 +71,7 @@ export class A {
 const modAModuleInfo = mockModuleInfo(
   "modA",
   "/root/project/modA.ts",
+  MediaType.TypeScript,
   modASource,
   undefined
 );
@@ -75,6 +86,7 @@ export class B {
 const modBModuleInfo = mockModuleInfo(
   "modB",
   "/root/project/modB.ts",
+  MediaType.TypeScript,
   modBSource,
   undefined
 );
@@ -107,21 +119,31 @@ const moduleMap: {
     "foo/bar.ts": mockModuleInfo(
       "/root/project/foo/bar.ts",
       "/root/project/foo/bar.ts",
+      MediaType.TypeScript,
       fooBarTsSource,
       null
     ),
     "foo/baz.ts": mockModuleInfo(
       "/root/project/foo/baz.ts",
       "/root/project/foo/baz.ts",
+      MediaType.TypeScript,
       fooBazTsSource,
       fooBazTsOutput
     ),
-    "modA.ts": modAModuleInfo
+    "modA.ts": modAModuleInfo,
+    "some.txt": mockModuleInfo(
+      "/root/project/some.txt",
+      "/root/project/some.text",
+      MediaType.Unknown,
+      "console.log();",
+      null
+    )
   },
   "/root/project/foo/baz.ts": {
     "./bar.ts": mockModuleInfo(
       "/root/project/foo/bar.ts",
       "/root/project/foo/bar.ts",
+      MediaType.TypeScript,
       fooBarTsSource,
       fooBarTsOutput
     )
@@ -131,6 +153,43 @@ const moduleMap: {
   },
   "/root/project/modB.ts": {
     "./modA.ts": modAModuleInfo
+  },
+  "/moduleKinds": {
+    "foo.ts": mockModuleInfo(
+      "foo",
+      "/moduleKinds/foo.ts",
+      MediaType.TypeScript,
+      "console.log('foo');",
+      undefined
+    ),
+    "foo.d.ts": mockModuleInfo(
+      "foo",
+      "/moduleKinds/foo.d.ts",
+      MediaType.TypeScript,
+      "console.log('foo');",
+      undefined
+    ),
+    "foo.js": mockModuleInfo(
+      "foo",
+      "/moduleKinds/foo.js",
+      MediaType.JavaScript,
+      "console.log('foo');",
+      undefined
+    ),
+    "foo.json": mockModuleInfo(
+      "foo",
+      "/moduleKinds/foo.json",
+      MediaType.Json,
+      "console.log('foo');",
+      undefined
+    ),
+    "foo.txt": mockModuleInfo(
+      "foo",
+      "/moduleKinds/foo.txt",
+      MediaType.JavaScript,
+      "console.log('foo');",
+      undefined
+    )
   }
 };
 
@@ -180,6 +239,7 @@ const osMock = {
       moduleCache[fileName] = mockModuleInfo(
         fileName,
         fileName,
+        MediaType.TypeScript,
         sourceCode,
         outputCode
       );
@@ -192,7 +252,7 @@ const osMock = {
         return moduleMap[containingFile][moduleSpecifier];
       }
     }
-    return mockModuleInfo(null, null, null, null);
+    return mockModuleInfo(null, null, null, null, null);
   },
   exit(code: number): never {
     throw new Error(`os.exit(${code})`);
@@ -405,6 +465,23 @@ test(function compilerResolveModule() {
   teardown();
 });
 
+test(function compilerResolveModuleUnknownMediaType() {
+  setup();
+  let didThrow = false;
+  try {
+    compilerInstance.resolveModule("some.txt", "/root/project");
+  } catch (e) {
+    assert(e instanceof Error);
+    assertEqual(
+      e.message,
+      `Unknown media type for: "some.txt" from "/root/project".`
+    );
+    didThrow = true;
+  }
+  assert(didThrow);
+  teardown();
+});
+
 test(function compilerGetModuleDependencies() {
   setup();
   const bazDeps = ["require", "exports", "./bar.ts"];
@@ -484,11 +561,33 @@ test(function compilerRecompileFlag() {
 });
 
 test(function compilerGetScriptKind() {
-  assertEqual(compilerInstance.getScriptKind("foo.ts"), ts.ScriptKind.TS);
-  assertEqual(compilerInstance.getScriptKind("foo.d.ts"), ts.ScriptKind.TS);
-  assertEqual(compilerInstance.getScriptKind("foo.js"), ts.ScriptKind.JS);
-  assertEqual(compilerInstance.getScriptKind("foo.json"), ts.ScriptKind.JSON);
-  assertEqual(compilerInstance.getScriptKind("foo.txt"), ts.ScriptKind.JS);
+  setup();
+  compilerInstance.resolveModule("foo.ts", "/moduleKinds");
+  compilerInstance.resolveModule("foo.d.ts", "/moduleKinds");
+  compilerInstance.resolveModule("foo.js", "/moduleKinds");
+  compilerInstance.resolveModule("foo.json", "/moduleKinds");
+  compilerInstance.resolveModule("foo.txt", "/moduleKinds");
+  assertEqual(
+    compilerInstance.getScriptKind("/moduleKinds/foo.ts"),
+    ts.ScriptKind.TS
+  );
+  assertEqual(
+    compilerInstance.getScriptKind("/moduleKinds/foo.d.ts"),
+    ts.ScriptKind.TS
+  );
+  assertEqual(
+    compilerInstance.getScriptKind("/moduleKinds/foo.js"),
+    ts.ScriptKind.JS
+  );
+  assertEqual(
+    compilerInstance.getScriptKind("/moduleKinds/foo.json"),
+    ts.ScriptKind.JSON
+  );
+  assertEqual(
+    compilerInstance.getScriptKind("/moduleKinds/foo.txt"),
+    ts.ScriptKind.JS
+  );
+  teardown();
 });
 
 test(function compilerGetScriptVersion() {
diff --git a/js/os.ts b/js/os.ts
index a4a7d3cc7..0d54de998 100644
--- a/js/os.ts
+++ b/js/os.ts
@@ -1,11 +1,18 @@
 // Copyright 2018 the Deno authors. All rights reserved. MIT license.
-import { ModuleInfo } from "./types";
 import * as msg from "gen/msg_generated";
 import { assert } from "./util";
 import * as util from "./util";
 import * as flatbuffers from "./flatbuffers";
 import { sendSync } from "./dispatch";
 
+interface CodeInfo {
+  moduleName: string | undefined;
+  filename: string | undefined;
+  mediaType: msg.MediaType;
+  sourceCode: string | undefined;
+  outputCode: string | undefined;
+}
+
 /** Exit the Deno process with optional exit code. */
 export function exit(exitCode = 0): never {
   const builder = flatbuffers.createBuilder();
@@ -20,7 +27,7 @@ export function exit(exitCode = 0): never {
 export function codeFetch(
   moduleSpecifier: string,
   containingFile: string
-): ModuleInfo {
+): CodeInfo {
   util.log("os.ts codeFetch", moduleSpecifier, containingFile);
   // Send CodeFetch message
   const builder = flatbuffers.createBuilder();
@@ -38,11 +45,14 @@ export function codeFetch(
   );
   const codeFetchRes = new msg.CodeFetchRes();
   assert(baseRes!.inner(codeFetchRes) != null);
+  // flatbuffers returns `null` for an empty value, this does not fit well with
+  // idiomatic TypeScript under strict null checks, so converting to `undefined`
   return {
-    moduleName: codeFetchRes.moduleName(),
-    filename: codeFetchRes.filename(),
-    sourceCode: codeFetchRes.sourceCode(),
-    outputCode: codeFetchRes.outputCode()
+    moduleName: codeFetchRes.moduleName() || undefined,
+    filename: codeFetchRes.filename() || undefined,
+    mediaType: codeFetchRes.mediaType(),
+    sourceCode: codeFetchRes.sourceCode() || undefined,
+    outputCode: codeFetchRes.outputCode() || undefined
   };
 }
 
diff --git a/js/types.ts b/js/types.ts
index 4e35d1227..954816811 100644
--- a/js/types.ts
+++ b/js/types.ts
@@ -1,14 +1,6 @@
 // Copyright 2018 the Deno authors. All rights reserved. MIT license.
 export type TypedArray = Uint8Array | Float32Array | Int32Array;
 
-// @internal
-export interface ModuleInfo {
-  moduleName: string | null;
-  filename: string | null;
-  sourceCode: string | null;
-  outputCode: string | null;
-}
-
 // tslint:disable:max-line-length
 // Following definitions adapted from:
 //   https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/node/index.d.ts
diff --git a/src/deno_dir.rs b/src/deno_dir.rs
index e8cc86b18..186dce1b8 100644
--- a/src/deno_dir.rs
+++ b/src/deno_dir.rs
@@ -5,6 +5,7 @@ use errors::DenoResult;
 use errors::ErrorKind;
 use fs as deno_fs;
 use http_util;
+use msg;
 use ring;
 use std;
 use std::fmt::Write;
@@ -109,21 +110,28 @@ impl DenoDir {
     self: &DenoDir,
     module_name: &str,
     filename: &str,
-  ) -> DenoResult<String> {
+  ) -> DenoResult<(String, msg::MediaType)> {
     let p = Path::new(filename);
+    // We write a special ".mime" file into the `.deno/deps` directory along side the
+    // cached file, containing just the media type.
+    let mut media_type_filename = filename.to_string();
+    media_type_filename.push_str(".mime");
+    let mt = Path::new(&media_type_filename);
 
     let src = if self.reload || !p.exists() {
       println!("Downloading {}", module_name);
-      let source = http_util::fetch_sync_string(module_name)?;
+      let (source, content_type) = http_util::fetch_sync_string(module_name)?;
       match p.parent() {
         Some(ref parent) => fs::create_dir_all(parent),
         None => Ok(()),
       }?;
       deno_fs::write_file(&p, source.as_bytes(), 0o666)?;
-      source
+      deno_fs::write_file(&mt, content_type.as_bytes(), 0o666)?;
+      (source, map_content_type(&p, Some(&content_type)))
     } else {
       let source = fs::read_to_string(&p)?;
-      source
+      let content_type = fs::read_to_string(&mt)?;
+      (source, map_content_type(&p, Some(&content_type)))
     };
     Ok(src)
   }
@@ -141,18 +149,20 @@ impl DenoDir {
     let use_extension = |ext| {
       let module_name = format!("{}{}", module_name, ext);
       let filename = format!("{}{}", filename, ext);
-      let source_code = if is_module_remote {
+      let (source_code, media_type) = if is_module_remote {
         self.fetch_remote_source(&module_name, &filename)?
       } else {
         assert_eq!(
           module_name, filename,
           "if a module isn't remote, it should have the same filename"
         );
-        fs::read_to_string(Path::new(&filename))?
+        let path = Path::new(&filename);
+        (fs::read_to_string(path)?, map_content_type(path, None))
       };
       return Ok(CodeFetchOutput {
         module_name: module_name.to_string(),
         filename: filename.to_string(),
+        media_type,
         source_code,
         maybe_output_code: None,
       });
@@ -215,6 +225,7 @@ impl DenoDir {
       Ok(output_code) => Ok(CodeFetchOutput {
         module_name: out.module_name,
         filename: out.filename,
+        media_type: out.media_type,
         source_code: out.source_code,
         maybe_output_code: Some(output_code),
       }),
@@ -324,6 +335,7 @@ fn test_get_cache_filename() {
 pub struct CodeFetchOutput {
   pub module_name: String,
   pub filename: String,
+  pub media_type: msg::MediaType,
   pub source_code: String,
   pub maybe_output_code: Option<String>,
 }
@@ -646,3 +658,157 @@ fn parse_local_or_remote(p: &str) -> Result<url::Url, url::ParseError> {
     Url::from_file_path(p).map_err(|_err| url::ParseError::IdnaError)
   }
 }
+
+fn map_file_extension(path: &Path) -> msg::MediaType {
+  match path.extension() {
+    None => msg::MediaType::Unknown,
+    Some(os_str) => match os_str.to_str() {
+      Some("ts") => msg::MediaType::TypeScript,
+      Some("js") => msg::MediaType::JavaScript,
+      Some("json") => msg::MediaType::Json,
+      _ => msg::MediaType::Unknown,
+    },
+  }
+}
+
+#[test]
+fn test_map_file_extension() {
+  assert_eq!(
+    map_file_extension(Path::new("foo/bar.ts")),
+    msg::MediaType::TypeScript
+  );
+  assert_eq!(
+    map_file_extension(Path::new("foo/bar.d.ts")),
+    msg::MediaType::TypeScript
+  );
+  assert_eq!(
+    map_file_extension(Path::new("foo/bar.js")),
+    msg::MediaType::JavaScript
+  );
+  assert_eq!(
+    map_file_extension(Path::new("foo/bar.json")),
+    msg::MediaType::Json
+  );
+  assert_eq!(
+    map_file_extension(Path::new("foo/bar.txt")),
+    msg::MediaType::Unknown
+  );
+  assert_eq!(
+    map_file_extension(Path::new("foo/bar")),
+    msg::MediaType::Unknown
+  );
+}
+
+// convert a ContentType string into a enumerated MediaType
+fn map_content_type(path: &Path, content_type: Option<&str>) -> msg::MediaType {
+  match content_type {
+    Some(content_type) => {
+      // sometimes there is additional data after the media type in
+      // Content-Type so we have to do a bit of manipulation so we are only
+      // dealing with the actual media type
+      let ct_vector: Vec<&str> = content_type.split(";").collect();
+      let ct: &str = ct_vector.first().unwrap();
+      match ct.to_lowercase().as_ref() {
+        "application/typescript"
+        | "text/typescript"
+        | "video/vnd.dlna.mpeg-tts"
+        | "video/mp2t" => msg::MediaType::TypeScript,
+        "application/javascript"
+        | "text/javascript"
+        | "application/ecmascript"
+        | "text/ecmascript"
+        | "application/x-javascript" => msg::MediaType::JavaScript,
+        "application/json" | "text/json" => msg::MediaType::Json,
+        "text/plain" => map_file_extension(path),
+        _ => {
+          debug!("unknown content type: {}", content_type);
+          msg::MediaType::Unknown
+        }
+      }
+    }
+    None => map_file_extension(path),
+  }
+}
+
+#[test]
+fn test_map_content_type() {
+  // Extension only
+  assert_eq!(
+    map_content_type(Path::new("foo/bar.ts"), None),
+    msg::MediaType::TypeScript
+  );
+  assert_eq!(
+    map_content_type(Path::new("foo/bar.d.ts"), None),
+    msg::MediaType::TypeScript
+  );
+  assert_eq!(
+    map_content_type(Path::new("foo/bar.js"), None),
+    msg::MediaType::JavaScript
+  );
+  assert_eq!(
+    map_content_type(Path::new("foo/bar.json"), None),
+    msg::MediaType::Json
+  );
+  assert_eq!(
+    map_content_type(Path::new("foo/bar.txt"), None),
+    msg::MediaType::Unknown
+  );
+  assert_eq!(
+    map_content_type(Path::new("foo/bar"), None),
+    msg::MediaType::Unknown
+  );
+
+  // Media Type
+  assert_eq!(
+    map_content_type(Path::new("foo/bar"), Some("application/typescript")),
+    msg::MediaType::TypeScript
+  );
+  assert_eq!(
+    map_content_type(Path::new("foo/bar"), Some("text/typescript")),
+    msg::MediaType::TypeScript
+  );
+  assert_eq!(
+    map_content_type(Path::new("foo/bar"), Some("video/vnd.dlna.mpeg-tts")),
+    msg::MediaType::TypeScript
+  );
+  assert_eq!(
+    map_content_type(Path::new("foo/bar"), Some("video/mp2t")),
+    msg::MediaType::TypeScript
+  );
+  assert_eq!(
+    map_content_type(Path::new("foo/bar"), Some("application/javascript")),
+    msg::MediaType::JavaScript
+  );
+  assert_eq!(
+    map_content_type(Path::new("foo/bar"), Some("text/javascript")),
+    msg::MediaType::JavaScript
+  );
+  assert_eq!(
+    map_content_type(Path::new("foo/bar"), Some("application/ecmascript")),
+    msg::MediaType::JavaScript
+  );
+  assert_eq!(
+    map_content_type(Path::new("foo/bar"), Some("text/ecmascript")),
+    msg::MediaType::JavaScript
+  );
+  assert_eq!(
+    map_content_type(Path::new("foo/bar"), Some("application/x-javascript")),
+    msg::MediaType::JavaScript
+  );
+  assert_eq!(
+    map_content_type(Path::new("foo/bar"), Some("application/json")),
+    msg::MediaType::Json
+  );
+  assert_eq!(
+    map_content_type(Path::new("foo/bar"), Some("text/json")),
+    msg::MediaType::Json
+  );
+  assert_eq!(
+    map_content_type(Path::new("foo/bar.ts"), Some("text/plain")),
+    msg::MediaType::TypeScript
+  );
+  assert_eq!(
+    map_content_type(Path::new("foo/bar.ts"), Some("foo/bar")),
+    msg::MediaType::Unknown
+  );
+}
diff --git a/src/http_util.rs b/src/http_util.rs
index 1c6079c08..9c5ff9529 100644
--- a/src/http_util.rs
+++ b/src/http_util.rs
@@ -4,9 +4,10 @@ use errors::{DenoError, DenoResult};
 use tokio_util;
 
 use futures::future::{loop_fn, Loop};
-use futures::{Future, Stream};
+use futures::{future, Future, Stream};
 use hyper;
 use hyper::client::{Client, HttpConnector};
+use hyper::header::CONTENT_TYPE;
 use hyper::Uri;
 use hyper_rustls;
 
@@ -27,7 +28,7 @@ pub fn get_client() -> Client<Connector, hyper::Body> {
 
 // The CodeFetch message is used to load HTTP javascript resources and expects a
 // synchronous response, this utility method supports that.
-pub fn fetch_sync_string(module_name: &str) -> DenoResult<String> {
+pub fn fetch_sync_string(module_name: &str) -> DenoResult<(String, String)> {
   let url = module_name.parse::<Uri>().unwrap();
   let client = get_client();
   // TODO(kevinkassimo): consider set a max redirection counter
@@ -63,11 +64,18 @@ pub fn fetch_sync_string(module_name: &str) -> DenoResult<String> {
         Ok(Loop::Break(response))
       })
   }).and_then(|response| {
-    response
+    let content_type = response
+      .headers()
+      .get(CONTENT_TYPE)
+      .map(|content_type| content_type.to_str().unwrap().to_string());
+    let body = response
       .into_body()
       .concat2()
       .map(|body| String::from_utf8(body.to_vec()).unwrap())
-      .map_err(|err| DenoError::from(err))
+      .map_err(|err| DenoError::from(err));
+    body.join(future::ok(content_type))
+  }).and_then(|(body_string, maybe_content_type)| {
+    future::ok((body_string, maybe_content_type.unwrap()))
   });
 
   tokio_util::block_on(fetch_future)
@@ -77,9 +85,11 @@ pub fn fetch_sync_string(module_name: &str) -> DenoResult<String> {
 fn test_fetch_sync_string() {
   // Relies on external http server. See tools/http_server.py
   tokio_util::init(|| {
-    let p = fetch_sync_string("http://127.0.0.1:4545/package.json").unwrap();
+    let (p, m) =
+      fetch_sync_string("http://127.0.0.1:4545/package.json").unwrap();
     println!("package.json len {}", p.len());
     assert!(p.len() > 1);
+    assert!(m == "application/json")
   });
 }
 
@@ -87,8 +97,10 @@ fn test_fetch_sync_string() {
 fn test_fetch_sync_string_with_redirect() {
   // Relies on external http server. See tools/http_server.py
   tokio_util::init(|| {
-    let p = fetch_sync_string("http://127.0.0.1:4546/package.json").unwrap();
+    let (p, m) =
+      fetch_sync_string("http://127.0.0.1:4546/package.json").unwrap();
     println!("package.json len {}", p.len());
     assert!(p.len() > 1);
+    assert!(m == "application/json")
   });
 }
diff --git a/src/msg.fbs b/src/msg.fbs
index 9aa9b069c..870da1c40 100644
--- a/src/msg.fbs
+++ b/src/msg.fbs
@@ -102,6 +102,13 @@ table CwdRes {
   cwd: string;
 }
 
+enum MediaType: byte {
+  JavaScript = 0,
+  TypeScript,
+  Json,
+  Unknown
+}
+
 table Base {
   cmd_id: uint32;
   sync: bool = true; // TODO(ry) Change default to false.
@@ -137,6 +144,7 @@ table CodeFetchRes {
   // is the location of the locally downloaded source code.
   module_name: string;
   filename: string;
+  media_type: MediaType;
   source_code: string;
   output_code: string; // Non-empty only if cached.
 }
diff --git a/src/ops.rs b/src/ops.rs
index 4adea46f6..83a719a29 100644
--- a/src/ops.rs
+++ b/src/ops.rs
@@ -253,6 +253,7 @@ fn op_code_fetch(
     let mut msg_args = msg::CodeFetchResArgs {
       module_name: Some(builder.create_string(&out.module_name)),
       filename: Some(builder.create_string(&out.filename)),
+      media_type: out.media_type,
       source_code: Some(builder.create_string(&out.source_code)),
       ..Default::default()
     };
diff --git a/tests/019_media_types.ts b/tests/019_media_types.ts
new file mode 100644
index 000000000..dbd951b3e
--- /dev/null
+++ b/tests/019_media_types.ts
@@ -0,0 +1,23 @@
+// When run against the test HTTP server, it will serve different media types
+// based on the URL containing `.t#.` strings, which exercises the different
+// mapping of media types end to end.
+
+// tslint:disable:max-line-length
+import { loaded as loadedTs1 } from "http://localhost:4545/tests/subdir/mt_text_typescript.t1.ts";
+import { loaded as loadedTs2 } from "http://localhost:4545/tests/subdir/mt_video_vdn.t2.ts";
+import { loaded as loadedTs3 } from "http://localhost:4545/tests/subdir/mt_video_mp2t.t3.ts";
+import { loaded as loadedJs1 } from "http://localhost:4545/tests/subdir/mt_text_javascript.j1.js";
+import { loaded as loadedJs2 } from "http://localhost:4545/tests/subdir/mt_application_ecmascript.j2.js";
+import { loaded as loadedJs3 } from "http://localhost:4545/tests/subdir/mt_text_ecmascript.j3.js";
+import { loaded as loadedJs4 } from "http://localhost:4545/tests/subdir/mt_application_x_javascript.j4.js";
+
+console.log(
+  "success",
+  loadedTs1,
+  loadedTs2,
+  loadedTs3,
+  loadedJs1,
+  loadedJs2,
+  loadedJs3,
+  loadedJs4
+);
diff --git a/tests/019_media_types.ts.out b/tests/019_media_types.ts.out
new file mode 100644
index 000000000..b127519b5
--- /dev/null
+++ b/tests/019_media_types.ts.out
@@ -0,0 +1,8 @@
+Downloading http://localhost:4545/tests/subdir/mt_text_typescript.t1.ts
+Downloading http://localhost:4545/tests/subdir/mt_video_vdn.t2.ts
+Downloading http://localhost:4545/tests/subdir/mt_video_mp2t.t3.ts
+Downloading http://localhost:4545/tests/subdir/mt_text_javascript.j1.js
+Downloading http://localhost:4545/tests/subdir/mt_application_ecmascript.j2.js
+Downloading http://localhost:4545/tests/subdir/mt_text_ecmascript.j3.js
+Downloading http://localhost:4545/tests/subdir/mt_application_x_javascript.j4.js
+success true true true true true true true
diff --git a/tests/error_004_missing_module.ts.out b/tests/error_004_missing_module.ts.out
index 03ebec0ac..64932ddd0 100644
--- a/tests/error_004_missing_module.ts.out
+++ b/tests/error_004_missing_module.ts.out
@@ -4,8 +4,8 @@ NotFound: Cannot resolve module "bad-module.ts" from "[WILDCARD]/tests/error_004
     at sendSync ([WILDCARD]/js/dispatch.ts:[WILDCARD])
     at Object.codeFetch ([WILDCARD]/js/os.ts:[WILDCARD])
     at DenoCompiler.resolveModule ([WILDCARD]/js/compiler.ts:[WILDCARD])
-    at DenoCompiler._resolveModuleName ([WILDCARD]/js/compiler.ts:[WILDCARD])
     at moduleNames.map.name ([WILDCARD]/js/compiler.ts:[WILDCARD])
     at Array.map (<anonymous>)
     at DenoCompiler.resolveModuleNames ([WILDCARD]/js/compiler.ts:[WILDCARD])
     at Object.compilerHost.resolveModuleNames ([WILDCARD]/third_party/node_modules/typescript/lib/typescript.js:[WILDCARD])
+    at resolveModuleNamesWorker ([WILDCARD]/third_party/node_modules/typescript/lib/typescript.js:[WILDCARD])
diff --git a/tests/error_005_missing_dynamic_import.ts.out b/tests/error_005_missing_dynamic_import.ts.out
index b050c7801..eb991de9d 100644
--- a/tests/error_005_missing_dynamic_import.ts.out
+++ b/tests/error_005_missing_dynamic_import.ts.out
@@ -4,8 +4,8 @@ NotFound: Cannot resolve module "bad-module.ts" from "[WILDCARD]/tests/error_005
     at sendSync ([WILDCARD]/js/dispatch.ts:[WILDCARD])
     at Object.codeFetch ([WILDCARD]/js/os.ts:[WILDCARD])
     at DenoCompiler.resolveModule ([WILDCARD]/js/compiler.ts:[WILDCARD])
-    at DenoCompiler._resolveModuleName ([WILDCARD]/js/compiler.ts:[WILDCARD])
     at moduleNames.map.name ([WILDCARD]/js/compiler.ts:[WILDCARD])
     at Array.map (<anonymous>)
     at DenoCompiler.resolveModuleNames ([WILDCARD]/js/compiler.ts:[WILDCARD])
     at Object.compilerHost.resolveModuleNames ([WILDCARD]/third_party/node_modules/typescript/lib/typescript.js:[WILDCARD])
+    at resolveModuleNamesWorker ([WILDCARD]/third_party/node_modules/typescript/lib/typescript.js:[WILDCARD])
diff --git a/tests/error_006_import_ext_failure.ts.out b/tests/error_006_import_ext_failure.ts.out
index 3abc6fa6d..f2b6aa599 100644
--- a/tests/error_006_import_ext_failure.ts.out
+++ b/tests/error_006_import_ext_failure.ts.out
@@ -4,8 +4,8 @@ NotFound: Cannot resolve module "./non-existent" from "[WILDCARD]/tests/error_00
     at sendSync ([WILDCARD]/js/dispatch.ts:[WILDCARD])
     at Object.codeFetch ([WILDCARD]/js/os.ts:[WILDCARD])
     at DenoCompiler.resolveModule ([WILDCARD]/js/compiler.ts:[WILDCARD])
-    at DenoCompiler._resolveModuleName ([WILDCARD]/js/compiler.ts:[WILDCARD])
     at moduleNames.map.name ([WILDCARD]/js/compiler.ts:[WILDCARD])
     at Array.map (<anonymous>)
     at DenoCompiler.resolveModuleNames ([WILDCARD]/js/compiler.ts:[WILDCARD])
     at Object.compilerHost.resolveModuleNames ([WILDCARD]/third_party/node_modules/typescript/lib/typescript.js:[WILDCARD])
+    at resolveModuleNamesWorker ([WILDCARD]/third_party/node_modules/typescript/lib/typescript.js:[WILDCARD])
diff --git a/tests/subdir/mt_application_ecmascript.j2.js b/tests/subdir/mt_application_ecmascript.j2.js
new file mode 100644
index 000000000..ec30e0595
--- /dev/null
+++ b/tests/subdir/mt_application_ecmascript.j2.js
@@ -0,0 +1,3 @@
+define(["exports"], function(exports) {
+  exports.loaded = true;
+});
diff --git a/tests/subdir/mt_application_x_javascript.j4.js b/tests/subdir/mt_application_x_javascript.j4.js
new file mode 100644
index 000000000..ec30e0595
--- /dev/null
+++ b/tests/subdir/mt_application_x_javascript.j4.js
@@ -0,0 +1,3 @@
+define(["exports"], function(exports) {
+  exports.loaded = true;
+});
diff --git a/tests/subdir/mt_javascript.js b/tests/subdir/mt_javascript.js
new file mode 100644
index 000000000..ec30e0595
--- /dev/null
+++ b/tests/subdir/mt_javascript.js
@@ -0,0 +1,3 @@
+define(["exports"], function(exports) {
+  exports.loaded = true;
+});
diff --git a/tests/subdir/mt_text_ecmascript.j3.js b/tests/subdir/mt_text_ecmascript.j3.js
new file mode 100644
index 000000000..ec30e0595
--- /dev/null
+++ b/tests/subdir/mt_text_ecmascript.j3.js
@@ -0,0 +1,3 @@
+define(["exports"], function(exports) {
+  exports.loaded = true;
+});
diff --git a/tests/subdir/mt_text_javascript.j1.js b/tests/subdir/mt_text_javascript.j1.js
new file mode 100644
index 000000000..ec30e0595
--- /dev/null
+++ b/tests/subdir/mt_text_javascript.j1.js
@@ -0,0 +1,3 @@
+define(["exports"], function(exports) {
+  exports.loaded = true;
+});
diff --git a/tests/subdir/mt_text_typescript.t1.ts b/tests/subdir/mt_text_typescript.t1.ts
new file mode 100644
index 000000000..e67d2a017
--- /dev/null
+++ b/tests/subdir/mt_text_typescript.t1.ts
@@ -0,0 +1 @@
+export const loaded = true;
diff --git a/tests/subdir/mt_video_mp2t.t3.ts b/tests/subdir/mt_video_mp2t.t3.ts
new file mode 100644
index 000000000..e67d2a017
--- /dev/null
+++ b/tests/subdir/mt_video_mp2t.t3.ts
@@ -0,0 +1 @@
+export const loaded = true;
diff --git a/tests/subdir/mt_video_vdn.t2.ts b/tests/subdir/mt_video_vdn.t2.ts
new file mode 100644
index 000000000..e67d2a017
--- /dev/null
+++ b/tests/subdir/mt_video_vdn.t2.ts
@@ -0,0 +1 @@
+export const loaded = true;
diff --git a/tools/http_server.py b/tools/http_server.py
index c96b070e6..011f3c31a 100755
--- a/tools/http_server.py
+++ b/tools/http_server.py
@@ -14,9 +14,33 @@ PORT = 4545
 REDIRECT_PORT = 4546
 
 
+class ContentTypeHandler(SimpleHTTPServer.SimpleHTTPRequestHandler):
+    def guess_type(self, path):
+        if ".t1." in path:
+            return "text/typescript"
+        if ".t2." in path:
+            return "video/vnd.dlna.mpeg-tts"
+        if ".t3." in path:
+            return "video/mp2t"
+        if ".j1." in path:
+            return "text/javascript"
+        if ".j2." in path:
+            return "application/ecmascript"
+        if ".j3." in path:
+            return "text/ecmascript"
+        if ".j4." in path:
+            return "application/x-javascript"
+        return SimpleHTTPServer.SimpleHTTPRequestHandler.guess_type(self, path)
+
+
 def server():
     os.chdir(root_path)  # Hopefully the main thread doesn't also chdir.
-    Handler = SimpleHTTPServer.SimpleHTTPRequestHandler
+    Handler = ContentTypeHandler
+    Handler.extensions_map.update({
+        ".ts": "application/typescript",
+        ".js": "application/javascript",
+        ".json": "application/json",
+    })
     SocketServer.TCPServer.allow_reuse_address = True
     s = SocketServer.TCPServer(("", PORT), Handler)
     print "Deno test server http://localhost:%d/" % PORT