# TypeScript loader for webpack

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<br />
<p align="center">
  <h3 align="center">ts-loader</h3>

  <p align="center">
    This is the TypeScript loader for webpack.
    <br />
    <br />
    <a href="https://github.com/TypeStrong/ts-loader/issues">Installation</a>
    ·
    <a href="https://github.com/TypeStrong/ts-loader/issues">Report Bug</a>
    ·
    <a href="https://github.com/TypeStrong/ts-loader/issues">Request Feature</a>
  </p>
</p>

## Table of Contents

<!-- toc -->

- [Getting Started](#getting-started)
  * [Installation](#installation)
  * [Running](#running)
  * [Examples](#examples)
  * [Faster Builds](#faster-builds)
  * [Yarn Plug’n’Play](#yarn-plugnplay)
  * [Babel](#babel)
  * [Parallelising Builds](#parallelising-builds)
  * [Compatibility](#compatibility)
  * [Configuration](#configuration)
    + [`devtool` / sourcemaps](#devtool--sourcemaps)
  * [Code Splitting and Loading Other Resources](#code-splitting-and-loading-other-resources)
  * [Declarations (.d.ts)](#declarations-dts)
  * [Failing the build on TypeScript compilation error](#failing-the-build-on-typescript-compilation-error)
  * [`baseUrl` / `paths` module resolution](#baseurl--paths-module-resolution)
  * [Options](#options)
  * [Loader Options](#loader-options)
    + [transpileOnly](#transpileonly)
    + [happyPackMode](#happypackmode)
    + [resolveModuleName and resolveTypeReferenceDirective](#resolvemodulename-and-resolvetypereferencedirective)
    + [getCustomTransformers](#getcustomtransformers)
    + [logInfoToStdOut](#loginfotostdout)
    + [logLevel](#loglevel)
    + [silent](#silent)
    + [ignoreDiagnostics](#ignorediagnostics)
    + [reportFiles](#reportfiles)
    + [compiler](#compiler)
    + [configFile](#configfile)
    + [colors](#colors)
    + [errorFormatter](#errorformatter)
    + [compilerOptions](#compileroptions)
    + [instance](#instance)
    + [appendTsSuffixTo](#appendtssuffixto)
    + [appendTsxSuffixTo](#appendtsxsuffixto)
    + [onlyCompileBundledFiles](#onlycompilebundledfiles)
    + [allowTsInNodeModules](#allowtsinnodemodules)
    + [context](#context)
    + [experimentalFileCaching](#experimentalfilecaching)
    + [projectReferences](#projectreferences)
  * [Usage with webpack watch](#usage-with-webpack-watch)
  * [Hot Module replacement](#hot-module-replacement)
- [Contributing](#contributing)
- [License](#license)

<!-- tocstop -->

## Getting Started

### Installation

```
yarn add ts-loader --dev
```

or

```
npm install ts-loader --save-dev
```

You will also need to install TypeScript if you have not already.

```
yarn add typescript --dev
```

or

```
npm install typescript --save-dev
```

### Running

Use webpack like normal, including `webpack --watch` and `webpack-dev-server`, or through another
build system using the [Node.js API](http://webpack.github.io/docs/node.js-api.html).

### Examples

We have a number of example setups to accommodate different workflows. Our examples can be found [here](examples/).

We probably have more examples than we need.  That said, here's a good way to get started:

- I want the simplest setup going.  Use "[vanilla](examples/vanilla)" `ts-loader`
- I want the fastest compilation that's available.  Use [fork-ts-checker-webpack-plugin](https://github.com/Realytics/fork-ts-checker-webpack-plugin).  It performs type checking in a separate process with `ts-loader` just handling transpilation.

### Faster Builds

As your project becomes bigger, compilation time increases linearly. It's because typescript's semantic checker has to inspect all files on every rebuild. The simple solution is to disable it by using the `transpileOnly: true` option, but doing so leaves you without type checking and *will not output declaration files*.

You probably don't want to give up type checking; that's rather the point of TypeScript. So what you can do is use the [fork-ts-checker-webpack-plugin](https://github.com/Realytics/fork-ts-checker-webpack-plugin). It runs the type checker on a separate process, so your build remains fast thanks to `transpileOnly: true` but you still have the type checking. Also, the plugin has several optimizations to make incremental type checking faster (AST cache, multiple workers).

If you'd like to see a simple setup take a look at [our simple example](examples/fork-ts-checker-webpack-plugin/). For a more complex setup take a look at our [more involved example](examples/react-babel-karma-gulp).

### Yarn Plug’n’Play

`ts-loader` supports [Yarn Plug’n’Play](https://yarnpkg.com/en/docs/pnp).  The recommended way to integrate is using the [pnp-webpack-plugin](https://github.com/arcanis/pnp-webpack-plugin#ts-loader-integration).

### Babel

`ts-loader` works very well in combination with [babel](https://babeljs.io/) and [babel-loader](https://github.com/babel/babel-loader). There is an [example](https://github.com/Microsoft/TypeScriptSamples/tree/master/react-flux-babel-karma) of this in the official [TypeScript Samples](https://github.com/Microsoft/TypeScriptSamples). Alternatively take a look at our own [example](examples/react-babel-karma-gulp).

### Parallelising Builds

It's possible to parallelise your builds. Historically this was useful from a performance perspective with webpack 2 / 3.  [With webpack 4+ there appears to be significantly less benefit and perhaps even cost.](https://blog.johnnyreilly.com/2018/12/you-might-not-need-thread-loader.html)

But if that's what you want to do, there's two ways to achieve this: [happypack](https://github.com/amireh/happypack) and [thread-loader](https://github.com/webpack-contrib/thread-loader). Both should be used in combination with [fork-ts-checker-webpack-plugin](https://github.com/Realytics/fork-ts-checker-webpack-plugin) for typechecking.)

To read more, look at [this post](https://medium.com/webpack/typescript-webpack-super-pursuit-mode-83cc568dea79) by [@johnny_reilly](https://twitter.com/johnny_reilly) on the webpack publication channel.

If you'd like find out further ways to improve your build using the watch API then take a look at [this post](https://medium.com/@kenneth_chau/speeding-up-webpack-typescript-incremental-builds-by-7x-3912ba4c1d15) by [@kenneth_chau](https://twitter.com/kenneth_chau).

### Compatibility

* TypeScript: 2.4.1+
* webpack: 4.x+ (please use `ts-loader` 3.x if you need webpack 2 or 3 support)
* node: 6.11.5 minimum (aligned with webpack 4)

A full test suite runs each night (and on each pull request). It runs both on [Linux](https://travis-ci.org/TypeStrong/ts-loader) and [Windows](https://ci.appveyor.com/project/JohnReilly/ts-loader), testing `ts-loader` against major releases of TypeScript. The test suite also runs against TypeScript@next (because we want to use it as much as you do).

If you become aware of issues not caught by the test suite then please let us know. Better yet, write a test and submit it in a PR!

### Configuration

1. Create or update `webpack.config.js` like so:

   ```javascript
   module.exports = {
     mode: "development",
     devtool: "inline-source-map",
     entry: "./app.ts",
     output: {
       filename: "bundle.js"
     },
     resolve: {
       // Add `.ts` and `.tsx` as a resolvable extension.
       extensions: [".ts", ".tsx", ".js"]
     },
     module: {
       rules: [
         // all files with a `.ts` or `.tsx` extension will be handled by `ts-loader`
         { test: /\.tsx?$/, loader: "ts-loader" }
       ]
     }
   };
   ```

2. Add a [`tsconfig.json`](https://www.typescriptlang.org/docs/handbook/tsconfig-json.html) file. (The one below is super simple; but you can tweak this to your hearts desire)

   ```json
   {
     "compilerOptions": {
       "sourceMap": true
     }
   }
   ```

The [tsconfig.json](http://www.typescriptlang.org/docs/handbook/tsconfig-json.html) file controls
TypeScript-related options so that your IDE, the `tsc` command, and this loader all share the
same options.

#### `devtool` / sourcemaps

If you want to be able to debug your original source then you can thanks to the magic of sourcemaps. There are 2 steps to getting this set up with `ts-loader` and webpack.

First, for `ts-loader` to produce **sourcemaps**, you will need to set the [tsconfig.json](http://www.typescriptlang.org/docs/handbook/tsconfig-json.html) option as `"sourceMap": true`.

Second, you need to set the `devtool` option in your `webpack.config.js` to support the type of sourcemaps you want. To make your choice have a read of the [`devtool` webpack docs](https://webpack.js.org/configuration/devtool/). You may be somewhat daunted by the choice available. You may also want to vary the sourcemap strategy depending on your build environment. Here are some example strategies for different environments:

* `devtool: 'inline-source-map'` - Solid sourcemap support; the best "all-rounder". Works well with karma-webpack (not all strategies do)
* `devtool: 'cheap-module-eval-source-map'` - Best support for sourcemaps whilst debugging.
* `devtool: 'source-map'` - Approach that plays well with UglifyJsPlugin; typically you might use this in Production

### Code Splitting and Loading Other Resources

Loading css and other resources is possible but you will need to make sure that
you have defined the `require` function in a [declaration file](https://www.typescriptlang.org/docs/handbook/writing-declaration-files.html).

```typescript
declare var require: {
  <T>(path: string): T;
  (paths: string[], callback: (...modules: any[]) => void): void;
  ensure: (
    paths: string[],
    callback: (require: <T>(path: string) => T) => void
  ) => void;
};
```

Then you can simply require assets or chunks per the [webpack documentation](https://webpack.js.org/guides/code-splitting/).

```javascript
require("!style!css!./style.css");
```

The same basic process is required for code splitting. In this case, you `import` modules you need but you
don't directly use them. Instead you require them at [split points](http://webpack.github.io/docs/code-splitting.html#defining-a-split-point). See [this example](test/comparison-tests/codeSplitting) and [this example](test/comparison-tests/es6codeSplitting) for more details.

[TypeScript 2.4 provides support for ECMAScript's new `import()` calls. These calls import a module and return a promise to that module.](https://blogs.msdn.microsoft.com/typescript/2017/06/12/announcing-typescript-2-4-rc/) This is also supported in webpack - details on usage can be found [here](https://webpack.js.org/guides/code-splitting-async/#dynamic-import-import-). Happy code splitting!

### Declarations (.d.ts)

To output a built .d.ts file, you can set `"declaration": true` in your tsconfig, and use the [DeclarationBundlerPlugin](https://www.npmjs.com/package/declaration-bundler-webpack-plugin) in your webpack config.

### Failing the build on TypeScript compilation error

The build **should** fail on TypeScript compilation errors as of webpack 2. If for some reason it does not, you can use the [webpack-fail-plugin](https://www.npmjs.com/package/webpack-fail-plugin).

For more background have a read of [this issue](https://github.com/TypeStrong/ts-loader/issues/108).

### `baseUrl` / `paths` module resolution

If you want to resolve modules according to `baseUrl` and `paths` in your `tsconfig.json` then you can use the [tsconfig-paths-webpack-plugin](https://www.npmjs.com/package/tsconfig-paths-webpack-plugin) package. For details about this functionality, see the [module resolution documentation](https://www.typescriptlang.org/docs/handbook/module-resolution.html#base-url).

This feature requires webpack 2.1+ and TypeScript 2.0+. Use the config below or check the [package](https://github.com/dividab/tsconfig-paths-webpack-plugin/blob/master/README.md) for more information on usage.

```javascript
const TsconfigPathsPlugin = require('tsconfig-paths-webpack-plugin');

module.exports = {
  ...
  resolve: {
    plugins: [new TsconfigPathsPlugin({ configFile: "./path/to/tsconfig.json" })]
  }
  ...
}
```

### Options

There are two types of options: TypeScript options (aka "compiler options") and loader options. TypeScript options should be set using a tsconfig.json file. Loader options can be specified through the `options` property in the webpack configuration:

```javascript
module.exports = {
  ...
  module: {
    rules: [
      {
        test: /\.tsx?$/,
        use: [
          {
            loader: 'ts-loader',
            options: {
              transpileOnly: true
            }
          }
        ]
      }
    ]
  }
}
```

### Loader Options

#### transpileOnly
| Type | Default Value |
|------|--------------|
| `boolean` | `false`|

If you want to speed up compilation significantly you can set this flag.
However, many of the benefits you get from static type checking between
different dependencies in your application will be lost.

It's advisable to use `transpileOnly` alongside the [fork-ts-checker-webpack-plugin](https://github.com/Realytics/fork-ts-checker-webpack-plugin) to get full type checking again. To see what this looks like in practice then either take a look at [our simple example](examples/fork-ts-checker-webpack-plugin). For a more complex setup take a look at our [more involved example](examples/react-babel-karma-gulp).

If you enable this option, webpack 4 will give you "export not found" warnings any time you re-export a type:

```
WARNING in ./src/bar.ts
1:0-34 "export 'IFoo' was not found in './foo'
 @ ./src/bar.ts
 @ ./src/index.ts
```

The reason this happens is that when typescript doesn't do a full type check, it does not have enough information to determine whether an imported name is a type or not, so when the name is then exported, typescript has no choice but to emit the export. Fortunately, the extraneous export should not be harmful, so you can just suppress these warnings:

```javascript
module.exports = {
  ...
  stats: {
    warningsFilter: /export .* was not found in/
  }
}
```

#### happyPackMode
| Type | Default Value |
|------|--------------|
| `boolean` | `false`|

If you're using [HappyPack](https://github.com/amireh/happypack) or [thread-loader](https://github.com/webpack-contrib/thread-loader) to parallise your builds then you'll need to set this to `true`. This implicitly sets `*transpileOnly*` to `true` and **WARNING!** stops registering **_all_** errors to webpack.

It's advisable to use this with the [fork-ts-checker-webpack-plugin](https://github.com/Realytics/fork-ts-checker-webpack-plugin) to get full type checking again. To see what this looks like in practice then either take a look at [our simple thread-loader example](examples/thread-loader). **_IMPORTANT_**: If you are using fork-ts-checker-webpack-plugin alongside HappyPack or thread-loader then ensure you set the `syntactic` diagnostic option like so:

```javascript
        new ForkTsCheckerWebpackPlugin({
          typescript: {
            diagnosticOptions: {
              semantic: true,
              syntactic: true,
            },
          },
        })
```

This 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 (as when used with `ts-loader` in `transpileOnly` mode, `ts-loader` will still report syntactic errors).

Also, if you are using `thread-loader` in watch mode, remember to set `poolTimeout: Infinity` so workers don't die.

#### resolveModuleName and resolveTypeReferenceDirective

These options should be functions which will be used to resolve the import statements and the `<reference types="...">` directives instead of the default TypeScript implementation. It's not intended that these will typically be used by a user of `ts-loader` - they exist to facilitate functionality such as [Yarn Plug’n’Play](https://yarnpkg.com/en/docs/pnp).

#### getCustomTransformers
| Type |
|------|
| ` (program: Program) => { before?: TransformerFactory<SourceFile>[]; after?: TransformerFactory<SourceFile>[]; } ` |

Provide custom transformers - only compatible with TypeScript 2.3+ (and 2.4 if using `transpileOnly` mode). For example usage take a look at [typescript-plugin-styled-components](https://github.com/Igorbek/typescript-plugin-styled-components) or our [test](test/comparison-tests/customTransformer).

You can also pass a path string to locate a js module file which exports the function described above, this useful especially in `happyPackMode`. (Because forked processes cannot serialize functions see more at [related issue](https://github.com/Igorbek/typescript-plugin-styled-components/issues/6#issue-303387183))

#### logInfoToStdOut
| Type | Default Value |
|------|--------------|
| `boolean` | `false`|

This is important if you read from stdout or stderr and for proper error handling.
The default value ensures that you can read from stdout e.g. via pipes or you use webpack -j to generate json output.

#### logLevel
| Type | Default Value |
|------|--------------|
| `string` | `warn` |

Can be `info`, `warn` or `error` which limits the log output to the specified log level.
Beware of the fact that errors are written to stderr and everything else is written to stderr (or stdout if logInfoToStdOut is true).

#### silent
| Type | Default Value |
|------|--------------|
| `boolean` | `false`|

If `true`, no console.log messages will be emitted. Note that most error
messages are emitted via webpack which is not affected by this flag.

#### ignoreDiagnostics
| Type | Default Value |
|------|--------------|
| `number[]` | `[]`|

You can squelch certain TypeScript errors by specifying an array of diagnostic
codes to ignore.

#### reportFiles
| Type | Default Value |
|------|--------------|
| `string[]` | `[]`|

Only report errors on files matching these glob patterns.

```javascript
  // in webpack.config.js
  {
    test: /\.ts$/,
    loader: 'ts-loader',
    options: { reportFiles: ['src/**/*.{ts,tsx}', '!src/skip.ts'] }
  }
```

This can be useful when certain types definitions have errors that are not fatal to your application.

#### compiler
| Type | Default Value |
|------|--------------|
| `string` | `'typescript'`|

Allows use of TypeScript compilers other than the official one. Should be
set to the NPM name of the compiler, eg [`ntypescript`](https://github.com/basarat/ntypescript).

#### configFile
| Type | Default Value |
|------|--------------|
| `string` | `'tsconfig.json'`|

Allows you to specify where to find the TypeScript configuration file.

You may provide

* just a file name. The loader then will search for the config file of each entry point in the respective entry point's containing folder. If a config file cannot be found there, it will travel up the parent directory chain and look for the config file in those folders.
* a relative path to the configuration file. It will be resolved relative to the respective `.ts` entry file.
* an absolute path to the configuration file.

Please note, that if the configuration file is outside of your project directory, you might need to set the `context` option to avoid TypeScript issues (like TS18003).
In this case the `configFile` should point to the `tsconfig.json` and `context` to the project root.

#### colors
| Type | Default Value |
|------|--------------|
| `boolean` | `true`|

If `false`, disables built-in colors in logger messages.

#### errorFormatter
| Type | Default Value |
|------|--------------|
| `(message: ErrorInfo, colors: boolean) => string` | `undefined`|

By default `ts-loader` formats TypeScript compiler output for an error or a warning in the style:

```
[tsl] ERROR in myFile.ts(3,14)
      TS4711: you did something very wrong
```

If that format is not to your taste you can supply your own formatter using the `errorFormatter` option. Below is a template for a custom error formatter. Please note that the `colors` parameter is an instance of [`chalk`](https://github.com/chalk/chalk) which you can use to color your output. (This instance will respect the `colors` option.)

```javascript
function customErrorFormatter(error, colors) {
  const messageColor =
    error.severity === "warning" ? colors.bold.yellow : colors.bold.red;
  return (
    "Does not compute.... " +
    messageColor(Object.keys(error).map(key => `${key}: ${error[key]}`))
  );
}
```

If the above formatter received an error like this:

```json
{
  "code":2307,
  "severity": "error",
  "content": "Cannot find module 'components/myComponent2'.",
  "file":"/.test/errorFormatter/app.ts",
  "line":2,
  "character":31
}
```

It would produce an error message that said:

```
Does not compute.... code: 2307,severity: error,content: Cannot find module 'components/myComponent2'.,file: /.test/errorFormatter/app.ts,line: 2,character: 31
```

And the bit after "Does not compute.... " would be red.

#### compilerOptions
| Type | Default Value |
|------|--------------|
| `object` | `{}`|

Allows overriding TypeScript options. Should be specified in the same format
as you would do for the `compilerOptions` property in tsconfig.json.

#### instance
| Type | Default Value |
|------|--------------|
| `string` | `TODO`|

Advanced option to force files to go through different instances of the
TypeScript compiler. Can be used to force segregation between different parts
of your code.

#### appendTsSuffixTo
| Type | Default Value |
|------|--------------|
| `RegExp[]` | `[]`|

#### appendTsxSuffixTo
| Type | Default Value |
|------|--------------|
| `RegExp[]` | `[]`|

A list of regular expressions to be matched against filename. If filename matches one of the regular expressions, a `.ts` or `.tsx` suffix will be appended to that filename.

This is useful for `*.vue` [file format](https://vuejs.org/v2/guide/single-file-components.html) for now. (Probably will benefit from the new single file format in the future.)

Example:

webpack.config.js:

```javascript
module.exports = {
  entry: "./index.vue",
  output: { filename: "bundle.js" },
  resolve: {
    extensions: [".ts", ".vue"]
  },
  module: {
    rules: [
      { test: /\.vue$/, loader: "vue-loader" },
      {
        test: /\.ts$/,
        loader: "ts-loader",
        options: { appendTsSuffixTo: [/\.vue$/] }
      }
    ]
  }
};
```

index.vue

```vue
<template><p>hello {{msg}}</p></template>
<script lang="ts">
export default {
  data(): Object {
    return {
      msg: "world"
    };
  }
};
</script>
```

We can handle `.tsx` by quite similar way:

webpack.config.js:

```javascript
module.exports = {
    entry: './index.vue',
    output: { filename: 'bundle.js' },
    resolve: {
        extensions: ['.ts', '.tsx', '.vue', '.vuex']
    },
    module: {
        rules: [
            { test: /\.vue$/, loader: 'vue-loader',
              options: {
                loaders: {
                  ts: 'ts-loader',
                  tsx: 'babel-loader!ts-loader',
                }
              }
            },
            { test: /\.ts$/, loader: 'ts-loader', options: { appendTsSuffixTo: [/TS\.vue$/] } }
            { test: /\.tsx$/, loader: 'babel-loader!ts-loader', options: { appendTsxSuffixTo: [/TSX\.vue$/] } }
        ]
    }
}
```

tsconfig.json (set `jsx` option to `preserve` to let babel handle jsx)

```json
{
  "compilerOptions": {
    "jsx": "preserve"
  }
}
```

index.vue

```vue
<script lang="tsx">
export default {
  functional: true,
  render(h, c) {
    return (<div>Content</div>);
  }
}
</script>
```

Or if you want to use only tsx, just use the `appendTsxSuffixTo` option only:

```javascript
            { test: /\.ts$/, loader: 'ts-loader' }
            { test: /\.tsx$/, loader: 'babel-loader!ts-loader', options: { appendTsxSuffixTo: [/\.vue$/] } }
```

#### onlyCompileBundledFiles
| Type | Default Value |
|------|--------------|
| `boolean` | `false`|

The default behavior of `ts-loader` is to act as a drop-in replacement for the `tsc` command,
so it respects the `include`, `files`, and `exclude` options in your `tsconfig.json`, loading
any files specified by those options. The `onlyCompileBundledFiles` option modifies this behavior,
loading only those files that are actually bundled by webpack, as well as any `.d.ts` files included
by the `tsconfig.json` settings. `.d.ts` files are still included because they may be needed for
compilation without being explicitly imported, and therefore not picked up by webpack.

#### allowTsInNodeModules
| Type | Default Value |
|------|--------------|
| `boolean` | `false`|

By default, `ts-loader` will not compile `.ts` files in `node_modules`.
You should not need to recompile `.ts` files there, but if you really want to, use this option.
Note that this option acts as a *whitelist* - any modules you desire to import must be included in
the `"files"` or `"include"` block of your project's `tsconfig.json`.

See: [https://github.com/Microsoft/TypeScript/issues/12358](https://github.com/Microsoft/TypeScript/issues/12358)

```javascript
  // in webpack.config.js
  {
    test: /\.ts$/,
    loader: 'ts-loader',
    options: { allowTsInNodeModules: true }
  }
```

And in your `tsconfig.json`:

```json
  {
    "include": [
      "node_modules/whitelisted_module.ts"
    ],
    "files": [
      "node_modules/my_module/whitelisted_file.ts"
    ]
  }
```

#### context
| Type | Default Value |
|------|--------------|
| `string` | `undefined`|

If set, will parse the TypeScript configuration file with given **absolute path** as base path.
Per default the directory of the configuration file is used as base path. Relative paths in the configuration
file are resolved with respect to the base path when parsed. Option `context` allows to set option
`configFile` to a path other than the project root (e.g. a NPM package), while the base path for `ts-loader`
can remain the project root.

Keep in mind that **not** having a `tsconfig.json` in your project root can cause different behaviour between `ts-loader` and `tsc`.
When using editors like `VS Code` it is advised to add a `tsconfig.json` file to the root of the project and extend the config file
referenced in option `configFile`. For more information please [read the PR](https://github.com/TypeStrong/ts-loader/pull/681) that
is the base and [read the PR](https://github.com/TypeStrong/ts-loader/pull/688) that contributed this option.

webpack:

```javascript
{
  loader: require.resolve('ts-loader'),
  options: {
    context: __dirname,
    configFile: require.resolve('ts-config-react-app')
  }
}
```

Extending `tsconfig.json`:

```json
{ "extends": "./node_modules/ts-config-react-app/index" }
```

Note that changes in the extending file while not be respected by `ts-loader`. Its purpose is to satisfy the code editor.

#### experimentalFileCaching
| Type | Default Value |
|------|--------------|
| `boolean` | `true`|

By default whenever the TypeScript compiler needs to check that a file/directory exists or resolve symlinks it makes syscalls. It does not cache the result of these operations and this may result in many syscalls with the same arguments ([see comment](https://github.com/TypeStrong/ts-loader/issues/825#issue-354725524) with example).
In some cases it may produce performance degradation.

This flag enables caching for some FS-functions like `fileExists`, `realpath` and `directoryExists` for TypeScript compiler. Note that caches are cleared between compilations.

#### projectReferences
| Type | Default Value |
|------|--------------|
| `boolean` | `false`|

ts-loader has opt-in support for [project references](https://www.typescriptlang.org/docs/handbook/project-references.html). With this configuration option enabled, `ts-loader` will incrementally rebuild upstream projects the same way `tsc --build` does. Otherwise, source files in referenced projects will be treated as if they’re part of the root project.

### Usage with webpack watch

Because TS will generate .js and .d.ts files, you should ignore these files, otherwise watchers may go into an infinite watch loop. For example, when using webpack, you may wish to add this to your webpack.conf.js file:

```javascript
 plugins: [
   new webpack.WatchIgnorePlugin([
     /\.js$/,
     /\.d\.ts$/
   ])
 ],
```

It's worth noting that use of the `LoaderOptionsPlugin` is [only supposed to be a stopgap measure](https://webpack.js.org/plugins/loader-options-plugin/). You may want to look at removing it entirely.

### Hot Module replacement

We do not support HMR as we did not yet work out a reliable way how to set it up.

If you want to give `webpack-dev-server` HMR a try, follow the official [webpack HMR guide](https://webpack.js.org/guides/hot-module-replacement/), then tweak a few config options for `ts-loader`:

1. Set `transpileOnly` to `true` (see [transpileOnly](#transpileonly) for config details and recommendations above).
2. Inside your HMR acceptance callback function, maybe re-require the module that was replaced.

## Contributing

This is your TypeScript loader! We want you to help make it even better. Please feel free to contribute; see the [contributor's guide](CONTRIBUTING.md) to get started.

## License

MIT License
