Runs typescript type checker and linter on separate process.
Last updated 18 days ago by piotr-oles .
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Fork TS Checker Webpack Plugin

Webpack plugin that runs TypeScript type checker on a separate process.

npm version npm beta version build status downloads commitizen friendly code style: prettier semantic-release


This plugin requires minimum webpack 2.3, TypeScript 2.1 and optionally ESLint 6.0.0 or TSLint 4.0

# with npm
npm install --save-dev fork-ts-checker-webpack-plugin

# with yarn
yarn add --dev fork-ts-checker-webpack-plugin

Basic webpack config (with ts-loader)

const ForkTsCheckerWebpackPlugin = require('fork-ts-checker-webpack-plugin');

const webpackConfig = {
  context: __dirname, // to automatically find tsconfig.json
  entry: './src/index.ts',
  module: {
    rules: [
        test: /\.tsx?$/,
        loader: 'ts-loader',
        options: {
          // disable type checker - we will use it in fork plugin
          transpileOnly: true
  plugins: [new ForkTsCheckerWebpackPlugin()]


There was already similar solution - awesome-typescript-loader. You can add CheckerPlugin and delegate checker to the separate process. The problem with awesome-typescript-loader was that, in our case, it was a lot slower than ts-loader on an incremental build (~20s vs ~3s). Secondly, we used tslint and we wanted to run this, along with type checker, in a separate process. This is why this plugin was created. To provide better performance, the plugin reuses Abstract Syntax Trees between compilations and shares these trees with TSLint. It can be scaled with a multi-process mode to utilize maximum CPU power.

Modules resolution

It's very important to be aware that this plugin uses TypeScript's, not webpack's modules resolution. It means that you have to setup tsconfig.json correctly. For example if you set files: ['./src/someFile.ts'] in tsconfig.json, this plugin will check only someFile.ts for semantic errors. It's because of performance. The goal of this plugin is to be as fast as possible. With TypeScript's module resolution we don't have to wait for webpack to compile files (which traverses dependency graph during compilation) - we have a full list of files from the begin.

To debug TypeScript's modules resolution, you can use tsc --traceResolution command.


ESLint is the future of linting in the TypeScript world. If you'd like to use eslint with the plugin, supply this option: eslint: true and ensure you have the relevant dependencies installed:

yarn add eslint @typescript-eslint/parser @typescript-eslint/eslint-plugin --dev

You should have an ESLint configuration file in your root project directory. Here is a sample .eslintrc.js configuration for a TypeScript project:

const path = require('path');
module.exports = {
  parser: '@typescript-eslint/parser', // Specifies the ESLint parser
  extends: [
    'plugin:@typescript-eslint/recommended' // Uses the recommended rules from the @typescript-eslint/eslint-plugin
  parserOptions: {
    project: path.resolve(__dirname, './tsconfig.json'),
    tsconfigRootDir: __dirname,
    ecmaVersion: 2018, // Allows for the parsing of modern ECMAScript features
    sourceType: 'module', // Allows for the use of imports
  rules: {
    // Place to specify ESLint rules. Can be used to overwrite rules specified from the extended configs
    // e.g. "@typescript-eslint/explicit-function-return-type": "off",

There's a good explanation on setting up TypeScript ESLint support by Robert Cooper here.


TSLint is being replaced by ESLint. As a consequence, support for TSLint in fork-ts-checker-webpack-plugin will be deprecated and removed in future versions of the plugin.

If you have installed tslint, you can enable it by setting tslint: true or tslint: './path/to/tslint.json'. We recommend changing defaultSeverity to a "warning" in tslint.json file. It helps to distinguish lints from TypeScript's diagnostics.


  • tsconfig string: Path to tsconfig.json file. Default: path.resolve(compiler.options.context, './tsconfig.json').

  • compilerOptions object: Allows overriding TypeScript options. Should be specified in the same format as you would do for the compilerOptions property in tsconfig.json. Default: {}.

  • eslint true | undefined:

    • If true, this activates eslint support.
  • eslintOptions object:

  • tslint string | true | undefined:

    • If string, path to tslint.json file to check source files against.

    • If true, path to tslint.json file will be computed with respect to currently checked file, just like TSLint CLI would do. Suppose you have a project:


      In such a case src/file.ts and src/anotherFile.ts would be checked against root tslint.json, and src/lib/someHelperFile.ts would be checked against src/lib/tslint.json.

    Default: undefined.

  • tslintAutoFix boolean: Passes on --fix flag while running tslint to auto fix linting errors. Default: false.

  • watch string | string[]: Directories or files to watch by service. Not necessary but improves performance (reduces number of fs.stat calls). Not applicable when useTypescriptIncrementalApi is true, in which case watching is handled automatically.

  • async boolean: True by default - async: false can block webpack's emit to wait for type checker/linter and to add errors to the webpack's compilation. We recommend to set this to false in projects where type checking is faster than webpack's build - it's better for integration with other plugins. Another scenario where you might want to set this to false is if you use the overlay functionality of webpack-dev-server.

  • ignoreDiagnostics number[]: List of TypeScript diagnostic codes to ignore.

  • ignoreLints string[]: List of tslint rule names to ignore.

  • ignoreLintWarnings boolean: If true, will ignore all lint warnings.

  • reportFiles string[]: Only report errors on files matching these glob patterns. This can be useful when certain types definitions have errors that are not fatal to your application. Default: []. Please note that this may behave unexpectedly if using the incremental API as the incremental API doesn't look for global and semantic errors if it has already found syntactic errors.

// in webpack.config.js
new ForkTsCheckerWebpackPlugin({
  reportFiles: ['src/**/*.{ts,tsx}', '!src/skip.ts']
  • colors boolean: If false, disables built-in colors in logger messages. Default: true.

  • logger object: Logger instance. It should be object that implements method: error, warn, info. Default: console.

  • formatter 'default' | 'codeframe' | ((message: NormalizedMessage, useColors: boolean) => string): Formatter for diagnostics and lints. By default uses default formatter. You can also pass your own formatter as a function (see src/NormalizedMessage.js and src/formatter/ for api reference).

  • formatterOptions object: Options passed to formatters (currently only codeframe - see available options)

  • silent boolean: If true, logger will not be used. Default: false.

  • checkSyntacticErrors boolean: This option is useful if you're using ts-loader in happyPackMode with HappyPack or thread-loader to parallelise your builds. If true it will ensure that the plugin checks for both syntactic errors (eg const array = [{} {}];) and semantic errors (eg const x: number = '1';). By default the plugin only checks for semantic errors. This is because when ts-loader is used in transpileOnly mode, ts-loader will still report syntactic errors. When used in happyPackMode it does not. Default: false.

  • memoryLimit number: Memory limit for service process in MB. If service exits with allocation failed error, increase this number. Default: 2048.

  • workers number: You can split type checking to a few workers to speed-up increment build. Be careful - if you don't want to increase build time, you should keep free 1 core for build and 1 core for a system (for example system with 4 CPUs should use max 2 workers). Second thing - node doesn't share memory between workers - keep in mind that memory usage will increase. Be aware that in some scenarios increasing workers number can increase checking time. Default: ForkTsCheckerWebpackPlugin.ONE_CPU.

  • vue boolean | { enabled: boolean, compiler: string }: If true or enabled: true, the linter and compiler will process VueJs single-file-component (.vue) files. See the Vue section further down for information on how to correctly setup your project.

  • useTypescriptIncrementalApi boolean: If true, the plugin will use incremental compilation API introduced in TypeScript 2.7. In this mode you can only have 1 worker, but if the changes in your code are small (like you normally have when you work in 'watch' mode), the compilation may be much faster, even compared to multi-threaded compilation. Defaults to true when working with TypeScript 3+ and false when below 3. The default can be overridden by directly specifying a value.

  • measureCompilationTime boolean: If true, the plugin will measure the time spent inside the compilation code. This may be useful to compare modes, especially if there are other loaders/plugins involved in the compilation. requires node 8+

  • typescript string: If supplied this is a custom path where typescript can be found. Defaults to require.resolve('typescript').

  • resolveModuleNameModule and resolveTypeReferenceDirectiveModule string: Both of those options refer to files on the disk that respectively export a resolveModuleName or a resolveTypeReferenceDirectiveModule function. These functions will be used to resolve the import statements and the <reference types="..."> directives instead of the default TypeScript implementation. Check the following code for an example of what those functions should look like:

    <summary>Code sample</summary>
    const { resolveModuleName } = require(`ts-pnp`);
    exports.resolveModuleName = (
    ) => {
      return resolveModuleName(
    exports.resolveTypeReferenceDirective = (
    ) => {
      return resolveModuleName(

Pre-computed consts:

  • ForkTsCheckerWebpackPlugin.ONE_CPU - always use one CPU
  • ForkTsCheckerWebpackPlugin.ALL_CPUS - always use all CPUs (will increase build time)
  • ForkTsCheckerWebpackPlugin.ONE_CPU_FREE - leave only one CPU for build (probably will increase build time)
  • ForkTsCheckerWebpackPlugin.TWO_CPUS_FREE - recommended - leave two CPUs free (one for build, one for system)

Different behaviour in watch mode

If you turn on webpacks watch mode the fork-ts-checker-notifier-webpack-plugin will take care of logging type errors, not webpack itself. That means if you set silent: true you won't see type errors in your console in watch mode.

You can either set silent: false to show the logging from fork-ts-checker-notifier-webpack-plugin or set async: false. Now webpack itself will log type errors again, but note that this can slow down your builds depending on the size of your project.


You may already be using the excellent webpack-notifier plugin to make build failures more obvious in the form of system notifications. There's an equivalent notifier plugin designed to work with the fork-ts-checker-webpack-plugin. It is the fork-ts-checker-notifier-webpack-plugin and can be found here. This notifier deliberately has a similar API as the webpack-notifier plugin to make migration easier.

Known Issue Watching Non-Emitting Files

At present there is an issue with the plugin regarding the triggering of type-checking when a change is made in a source file that will not emit js. If you have a file which contains only interfaces and / or types then changes to it will not trigger the type checker whilst in watch mode. Sorry about that.

We hope this will be resolved in future; the issue can be tracked here.

Plugin Hooks

This plugin provides some custom webpack hooks (all are sync):

Event name Hook Access Key Description Params
fork-ts-checker-cancel cancel Cancellation has been requested cancellationToken
fork-ts-checker-waiting waiting Waiting for results hasTsLint
fork-ts-checker-service-before-start serviceBeforeStart Async plugin that can be used for delaying fork-ts-checker-service-start -
fork-ts-checker-service-start serviceStart Service will be started tsconfigPath, tslintPath, watchPaths, workersNumber, memoryLimit
fork-ts-checker-service-start-error serviceStartError Cannot start service error
fork-ts-checker-service-out-of-memory serviceOutOfMemory Service is out of memory -
fork-ts-checker-receive receive Plugin receives diagnostics and lints from service diagnostics, lints
fork-ts-checker-emit emit Service will add errors and warnings to webpack compilation ('build' mode) diagnostics, lints, elapsed
fork-ts-checker-done done Service finished type checking and webpack finished compilation ('watch' mode) diagnostics, lints, elapsed

The Event name is there for backward compatibility with webpack 2/3. Regardless of the version of webpack (2, 3 or 4) you are using, we will always access plugin hooks with Hook Access Keys as described below.

Accessing plugin hooks

All plugin hooks are compatible with both webpack version 4 and version 2. To access plugin hooks and tap into the event, we need to use the getCompilerHooks static method. When we call this method with a webpack compiler instance, it returns the series of tapable hooks where you can pass in your callbacks.

// require the plugin
const ForkTsCheckerWebpackPlugin = require('fork-ts-checker-webpack-plugin');
// setup compiler with the plugin
const compiler = webpack({
  // .. webpack config
// Optionally add the plugin to the compiler
// **Don't do this if already added through configuration**
new ForkTsCheckerWebpackPlugin({
  silent: true,
  async: true
// Now get the plugin hooks from compiler
const tsCheckerHooks = ForkTsCheckerWebpackPlugin.getCompilerHooks(compiler);
// These hooks provide access to different events
// =================================================== //
// The properties of tsCheckerHooks corresponds to the //
// Hook Access Key of the table above.                 //
// =================================================== //
// Example, if we want to run some code when plugin has received diagnostics
// and lint
tsCheckerHooks.receive.tap('yourListenerName', (diagnostics, lint) => {
  // do something with diagnostics, perhaps show custom message
// Say we want to show some message when plugin is waiting for typecheck results
tsCheckerHooks.waiting.tap('yourListenerName', () => {
  console.log('waiting for typecheck results');

Calling .tap() on any hooks, requires two arguments.

name (string)

The first argument passed to .tap is the name of your listener callback (yourListenerName). It doesn't need to correspond to anything special. It is intended to be used internally as the name of the hook.

callback (function)

The second argument is the callback function. Depending on the hook you are tapping into, several arguments are passed to the function. Do check the table above to find out which arguments are passed to which hooks.

Accessing hooks on Webpack Multi-Compiler instance

The above method will not work on webpack multi compiler instance. The reason is getCompilerHooks expects (at lease as of now) the same compiler instance to be passed where the plugin was attached. So in case of multi compiler, we need to access individual compiler instances.

const ForkTsCheckerWebpackPlugin = require('fork-ts-checker-webpack-plugin');
// setup multi compiler with the plugin
const compiler = webpack([
    // .. webpack config
    // .. webpack config

// safely determine if instance is multi-compiler
if ('compilers' in compiler) {
  compiler.compilers.forEach(singleCompiler => {
    // get plugin hooks from the single compiler instance
    const tsCheckerHooks = ForkTsCheckerWebpackPlugin.getCompilerHooks(
    // now access hooks just like before
    tsCheckerHooks.waiting.tap('yourListenerName', () => {
      console.log('waiting for typecheck results');


  1. Turn on the vue option in the plugin in your webpack config:
new ForkTsCheckerWebpackPlugin({
  tslint: true,
  vue: true

Optionally change default vue-template-compiler to nativescript-vue-template-compiler if you use nativescript-vue

new ForkTsCheckerWebpackPlugin({
  tslint: true,
  vue: { enabled: true, compiler: 'nativescript-vue-template-compiler' }
  1. To activate TypeScript in your .vue files, you need to ensure your script tag's language attribute is set to ts or tsx (also make sure you include the .vue extension in all your import statements as shown below):
<script lang="ts">
  import Hello from '@/components/hello.vue';

  // ...
  1. Ideally you are also using ts-loader (in transpileOnly mode). Your Webpack config rules may look something like this:
  test: /\.ts$/,
  loader: 'ts-loader',
  include: [resolve('src'), resolve('test')],
  options: {
    appendTsSuffixTo: [/\.vue$/],
    transpileOnly: true
  test: /\.vue$/,
  loader: 'vue-loader',
  options: vueLoaderConfig
  1. Add rules to your tslint.json and they will be applied to Vue files. For example, you could apply the Standard JS rules tslint-config-standard like this:
  "defaultSeverity": "error",
  "extends": ["tslint-config-standard"]
  1. Ensure your tsconfig.json includes .vue files:
// tsconfig.json
  "include": [
  "exclude": [
  1. It accepts any wildcard in your TypeScript configuration:
// tsconfig.json
  "compilerOptions": {

    // ...

    "baseUrl": ".",
    "paths": {
      "@/*": [
      "~/*": [

// In a .ts or .vue file...
import Hello from '@/components/hello.vue'
  1. If you are working in VSCode, you can get extensions Vetur and TSLint Vue to complete the developer workflow.


This plugin was created in Realytics in 2017. Thank you for supporting Open Source.


MIT License

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