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Fork TS Checker Webpack Plugin

Npm version Build Status

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


This plugin requires minimum webpack 2.3, typescript 2.1 and optionally tslint 5.0

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

Basic webpack config (with ts-loader)

var ForkTsCheckerWebpackPlugin = require('fork-ts-checker-webpack-plugin');
var 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 is 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 use tslint and we wanted to run this, along with type checker, in a separate process. This is why we've created this plugin. To provide better performance, 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.


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').

  • tslint string | true: Path to tslint.json file or true. If true, uses path.resolve(compiler.options.context, './tslint.json'). Default: undefined.

  • watch string | string[]: Directories or files to watch by service. Not necessary but improves performance (reduces number of fs.stat calls).

  • 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 use it in projects where type checking is faster than webpack's build - it's better for integration with other plugins.

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

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

  • 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' | Function: Formatter for diagnostics and lints. By default uses default formatter. You can also pass your own formatter as a function (see lib/NormalizedMessage.js and lib/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. 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.

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)


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