Debugging Language Service in VS Code

VS Code is designed around an extension model. This means that the client-side (ie: text-editor) code for communicating with the TypeScript server lives in "extensions/typescript" in the VS Code repo.¹ The server side code lives in src/services of the TypeScript repo.

We will use a stable version of vscode to debug a development version of vs code running against a development version of tsserver.

1. Download/install a stable version of vs code.

2. Follow the instructions to setup a development version of vs code.¹

3. Clone the typescript repo locally, and follow the instructions on building typescript.

4. Update your user settings to use your development version of typescript, located in the .../TypeScript/built/local directory. The corresponding setting/path is

{
   "typescript.tsdk": "/path/to/repo/TypeScript/built/local"
}

From here, there are different steps for debugging the client- and server-side, respectively.

Debugging tsserver (server-side)

Note: the gulp-build doesn't currently produce working source-maps, and building with jake may require some extra effort to fix the source-maps.

1. Set the ts-server to be open in debug mode on the right port using either of the following two methods (in the rest of this guide, we assume you chose 5859):

a. In a shell, export the TSS_DEBUG environment variable to an open port (you will run the development vs code instance from within that shell).

b. Edit extensions/typescript/src/typescriptServiceClient.ts, setting the port to an open one.

2. Update launch.json with an option to attach to the node instance, with sourcemaps from your built/local folder. You can use this option as a template:

		{
			"name": "Attach to TS Server",
			"type": "node",
			"request": "attach",
			"port": 5859,
			"sourceMaps": true,
			"outDir": "/path/to/repo/TypeScript/built/local"
		},

3. Launch an instance of development vs code, and open a ts file.

4. Launch an instance of stable vs code.

5. Attach the stable vs code instance to the development instance.

Debugging the Extension Host (client-side)

3. Launch an instance of development vs code.

4. Launch an instance of stable vs code.

5. Attach the stable vs code instance to the development instance.

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¹ In particular, the built-in extension spawns the node instance that loads tsserver via the call to electron.fork() in extensions/typescript/src/typescriptServiceClient.ts.

² If you are on Linux, be sure to increase the number of file watchers per the fix for ENOSPC errors. for opening medium-large projects like Typescript, the default limit of 8192 is almost certainly too small.