Writing a TypeScript Language Service Plugin

In TypeScript 2.2 and later, developers can enable language service plugins to augment the TypeScript code editing experience. The purpose of this guide is to help you write your own plugin.

What's a Language Service Plugin?

TypeScript Language Service Plugins ("plugins") are for changing the editing experience only. The core TypeScript language remains the same. Plugins can't add new language features such as new syntax or different typechecking behavior, and plugins aren't loaded during normal commandline typechecking or emitting.

Instead, plugins are for augmenting the editing experience. Some examples of things plugins might do:
* Provide errors from a linter inline in the editor
* Filter the completion list to remove certain properties from window
* Redirect "Go to definition" to go to a different location for certain identifiers
* Enable new errors or completions in string literals for a custom templating language

Examples of things language plugins cannot do:
* Add new custom syntax to TypeScript
* Change how the compiler emits JavaScript
* Customize the type system to change what is or isn't an error when running tsc

Developers using the plugin will npm install --save-dev your_plugin_name and edit their tsconfig.json file to enable your plugin.

Overview: Writing a Simple Plugin

Let's write a simple plugin. Our plugin will remove a user-configurable list of property names from the completion list. You might use this sort of plugin on your team to help remind you which APIs are 'banned' (for example, using the caller property of function is discouraged).

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When your plugin is loaded, it's first initialized as a factory function with its first parameter set to {typescript: ts}. It's important to use this value, rather than the imported ts module, because any version of TypeScript might be loaded by tsserver. If you use any other object, you'll run into compatibility problems later because enum values may change between versions.

Here's the minimal code that handles this injected ts value:
``ts
import * as ts_module from "../node_modules/typescript/lib/tsserverlibrary";

function init(modules: {typescript: typeof ts_module}) {
const ts = modules.typescript;
/ More to come here /
}

export = init;
`

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TypeScript Language Service Plugins use the Decorator Pattern to "wrap" the main TypeScript Language Service. When your plugin is initialized, it will be given a Language Service instance to wrap, and should return a new decorator wrapping this instance. This is exposed through the create function returned from your outer factory function.

Let's fill in some more code to properly set up a decorator:
`ts
function init(modules: {typescript: typeof ts_module}) {
const ts = modules.typescript;

function create(info: ts.server.PluginCreateInfo) {
// Set up decorator
const proxy = Object.create(null) as ts.LanguageService;
const oldLS = info.languageService;
for (const k in oldLS) {
(proxy)[k] = function () {
return oldLS[k].apply(oldLS, arguments);
}
}
return proxy;
}

return { create };
}
`

This sets up a "pass-through" decorator that invokes the underlying language service for all methods.

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To enable this plugin, users will add an entry to the plugins list in their tsconfig.json file:
`json
{
"compilerOptions": {
"noImplicitAny": true,
"plugins": [{ "name": "sample-ts-plugin" }]
}
}
`

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Let's modify the above pass-through plugin to add some new behavior.

We'll change the getCompletionsAtPosition function to remove certain entries named caller from the completion list:
`ts
// Remove specified entries from completion list
proxy.getCompletionsAtPosition = (fileName, position) => {
const prior = info.languageService.getCompletionsAtPosition(fileName, position);
prior.entries = prior.entries.filter(e => e.name !== 'caller');
return prior;
};
`

Handling User Configuration

Users can customize your plugin behavior by providing additional data in their tsconfig.json file. Your plugin is given its enabling entry from the tsconfig.json file in the info.config property.

Let's allow the user to customize the list of names to remove from the completion list:
`ts
function create(info: ts.server.PluginCreateInfo) {
// Get a list of things to remove from the completion list from the config object.
// If nothing was specified, we'll just remove 'caller'
const whatToRemove: string[] = info.config.remove || ['caller'];

// ... (set up decorator here) ...

// Remove specified entries from completion list
proxy.getCompletionsAtPosition = (fileName, position) => {
const prior = info.languageService.getCompletionsAtPosition(fileName, position);
prior.entries = prior.entries.filter(e => whatToRemove.indexOf(e.name) < 0);
return prior;
};
`

The new tsconfig.json file might look like this:
`json
{
"compilerOptions": {
"noImplicitAny": true,
"plugins": [{
"name": "sample-ts-plugin",
"remove": ["caller", "callee", "getDay"]
}]
}
}
`

Debugging

You'll probably want to add some logging to your plugin to help you during development. The TypeScript Server Log allows plugins to write to a common log file.

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Your plugin code runs inside the TypeScript Server process. Its logging behavior can be enabled by setting the TSS_LOG environment variable. To log to a file, set TSS_LOG to:
`
-logToFile true -file C:\SomeFolder\MyTypeScriptLog.txt -level verbose
`
Ensure that the containing directory (C:\SomeFolder in this example) exists and is writable.

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You can write to this log by calling into the TypeScript project's logging service:
`ts
function create(info: ts.server.PluginCreateInfo) {
info.project.projectService.logger.info("I'm getting set up now! Check the log for this message.");
`

Putting it all together

`ts
import * as ts_module from "../node_modules/typescript/lib/tsserverlibrary";

function init(modules: {typescript: typeof ts_module}) {
const ts = modules.typescript;

function create(info: ts.server.PluginCreateInfo) {
// Get a list of things to remove from the completion list from the config object.
// If nothing was specified, we'll just remove 'caller'
const whatToRemove: string[] = info.config.remove || ['caller'];

// Diagnostic logging
info.project.projectService.logger.info("I'm getting set up now! Check the log for this message.");

// Set up decorator
const proxy = Object.create(null) as ts.LanguageService;
const oldLS = info.languageService;
for (const k in oldLS) {
(proxy)[k] = function () {
return oldLS[k].apply(oldLS, arguments);
}
}

// Remove specified entries from completion list
proxy.getCompletionsAtPosition = (fileName, position) => {
const prior = info.languageService.getCompletionsAtPosition(fileName, position);
const oldLength = prior.entries.length;
prior.entries = prior.entries.filter(e => whatToRemove.indexOf(e.name) < 0);

// Sample logging for diagnostic purposes
if (oldLength !== prior.entries.length) {
info.project.projectService.logger.info(Removed ${oldLength - prior.entries.length} entries from the completion list);
}

return prior;
};

return proxy;
}

return { create };
}

export = init;
`

Testing Locally

Local testing of your plugin is similar to testing other node modules. To set up a sample project where you can easily test plugin changes:

* Run npm link from your plugin directory
* In your sample project, run npm link your_plugin_name
* Add an entry to the plugins field of the tsconfig.json`
* Rebuild your plugin and restart your editor to pick up code changes

Real-world Plugins

Some other TypeScript Language Service Plugin implementations you can look at for reference:
* https://github.com/angular/angular/blob/master/modules/%40angular/language-service/src/ts_plugin.ts