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1 # TypeScript loader for webpack
3 [ ![npm version ]( https://img.shields.io/npm/v/ts-loader.svg )]( https://www.npmjs.com/package/ts-loader )
4 [ ![Linux Build Status ]( https://travis-ci.org/TypeStrong/ts-loader.svg?branch=master )]( https://travis-ci.org/TypeStrong/ts-loader )
5 [ ![Windows Build Status ]( https://ci.appveyor.com/api/projects/status/bjh0r0d4ckspgkh9/branch/master?svg=true )]( https://ci.appveyor.com/project/JohnReilly/ts-loader/branch/master )
6 [ ![Downloads ]( http://img.shields.io/npm/dm/ts-loader.svg )]( https://npmjs.org/package/ts-loader )
7 [ ![node version ]( https://img.shields.io/node/v/ts-loader.svg )]( https://www.npmjs.com/package/ts-loader )
8 [ ![code style: prettier ]( https://img.shields.io/badge/code_style-prettier-ff69b4.svg )]( https://github.com/prettier/prettier )
9 [ ![Join the chat at https://gitter.im/TypeStrong/ts-loader ]( https://img.shields.io/badge/gitter-join%20chat-brightgreen.svg )]( https://gitter.im/TypeStrong/ts-loader )
11 This is the TypeScript loader for webpack.
13 ## Getting Started
15 ### Examples
17 We have a number of example setups to accommodate different workflows. Our examples can be found [ here ]( examples/ ).
19 We probably have more examples than we need. That said, here's a good way to get started:
21 - I want the simplest setup going. Use "[ vanilla ]( examples/vanilla )" ts-loader
22 - 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.
24 ### Faster Builds
26 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* .
28 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).
30 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 ).
32 ### Yarn Plug’n’Play
34 `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 ).
36 ### Babel
38 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 ).
40 ### Parallelising Builds
42 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 )
44 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.)
46 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.
48 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 ).
50 ### Installation
52 ```
53 yarn add ts-loader --dev
54 ```
56 or
58 ```
59 npm install ts-loader --save-dev
60 ```
62 You will also need to install TypeScript if you have not already.
64 ```
65 yarn add typescript --dev
66 ```
68 or
70 ```
71 npm install typescript --save-dev
72 ```
74 ### Running
76 Use webpack like normal, including `webpack --watch` and `webpack-dev-server` , or through another
77 build system using the [ Node.js API ]( http://webpack.github.io/docs/node.js-api.html ).
79 ### Compatibility
81 * TypeScript: 2.4.1+
82 * webpack: 4.x+ (please use ts-loader 3.x if you need webpack 2 or 3 support)
83 * node: 6.11.5 minimum (aligned with webpack 4)
85 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).
87 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!
89 ### Configuration
91 1. Create or update `webpack.config.js` like so:
93 ```javascript
94 module.exports = {
95 mode: "development",
96 devtool: "inline-source-map",
97 entry: "./app.ts",
98 output: {
99 filename: "bundle.js"
100 },
101 resolve: {
102 // Add `.ts` and `.tsx` as a resolvable extension.
103 extensions: [".ts", ".tsx", ".js"]
104 },
105 module: {
106 rules: [
107 // all files with a `.ts` or `.tsx` extension will be handled by `ts-loader`
108 { test: /\.tsx?$/, loader: "ts-loader" }
109 ]
110 }
111 };
112 ```
114 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)
116 ```json
117 {
118 "compilerOptions": {
119 "sourceMap": true
120 }
121 }
122 ```
124 The [tsconfig.json](http://www.typescriptlang.org/docs/handbook/tsconfig-json.html) file controls
125 TypeScript-related options so that your IDE, the `tsc` command, and this loader all share the
126 same options.
128 #### `devtool` / sourcemaps
130 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.
132 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`.
134 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:
136 * `devtool: 'inline-source-map'` - Solid sourcemap support; the best "all-rounder". Works well with karma-webpack (not all strategies do)
137 * `devtool: 'cheap-module-eval-source-map'` - Best support for sourcemaps whilst debugging.
138 * `devtool: 'source-map'` - Approach that plays well with UglifyJsPlugin; typically you might use this in Production
140 ### Code Splitting and Loading Other Resources
142 Loading css and other resources is possible but you will need to make sure that
143 you have defined the `require` function in a [declaration file](https://www.typescriptlang.org/docs/handbook/writing-declaration-files.html).
145 ```typescript
146 declare var require: {
147 <T>(path: string): T;
148 (paths: string[], callback: (...modules: any[]) => void): void;
149 ensure: (
150 paths: string[],
151 callback: (require: <T>(path: string) => T) => void
152 ) => void;
153 };
154 ```
156 Then you can simply require assets or chunks per the [ webpack documentation ]( https://webpack.js.org/guides/code-splitting/ ).
158 ```javascript
159 require("!style!css!./style.css");
160 ```
162 The same basic process is required for code splitting. In this case, you `import` modules you need but you
163 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.
165 [ 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!
167 ### Declarations (.d.ts)
169 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.
171 ### Failing the build on TypeScript compilation error
173 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 ).
175 For more background have a read of [ this issue ]( https://github.com/TypeStrong/ts-loader/issues/108 ).
177 ### `baseUrl` / `paths` module resolution
179 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 ).
181 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.
183 ```javascript
184 const TsconfigPathsPlugin = require('tsconfig-paths-webpack-plugin');
186 module.exports = {
187 ...
188 resolve: {
189 plugins: [new TsconfigPathsPlugin({ /*configFile: "./path/to/tsconfig.json" */ })]
190 }
191 ...
192 }
193 ```
195 ### Options
197 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:
199 ```javascript
200 module.exports = {
201 ...
202 module: {
203 rules: [
204 {
205 test: /\.tsx?$/,
206 use: [
207 {
208 loader: 'ts-loader',
209 options: {
210 transpileOnly: true
211 }
212 }
213 ]
214 }
215 ]
216 }
217 }
218 ```
220 ### Loader Options
222 #### transpileOnly _(boolean) (default=false)_
224 If you want to speed up compilation significantly you can set this flag.
225 However, many of the benefits you get from static type checking between
226 different dependencies in your application will be lost.
228 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 ).
230 If you enable this option, webpack 4 will give you "export not found" warnings any time you re-export a type:
232 ```
233 WARNING in ./src/bar.ts
234 1:0-34 "export 'IFoo' was not found in './foo'
235 @ ./src/bar.ts
236 @ ./src/index.ts
237 ```
239 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:
241 ```javascript
242 module.exports = {
243 ...
244 stats: {
245 warningsFilter: /export .* was not found in/
246 }
247 }
248 ```
250 #### happyPackMode _(boolean) (default=false)_
252 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.
254 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 HappyPack example ]( examples/happypack ) / [ our simple thread-loader example ]( examples/thread-loader ). For a more complex setup take a look at our [ more involved HappyPack example ]( examples/react-babel-karma-gulp-happypack ) / [ more involved thread-loader example ]( examples/react-babel-karma-gulp-thread-loader ). **_IMPORTANT_* *: If you are using fork-ts-checker-webpack-plugin alongside HappyPack or thread-loader then ensure you set the `checkSyntacticErrors` option like so:
256 ```javascript
257 new ForkTsCheckerWebpackPlugin({ checkSyntacticErrors: true })
258 ```
260 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).
262 Also, if you are using `thread-loader` in watch mode, remember to set `poolTimeout: Infinity` so workers don't die.
264 #### resolveModuleName and resolveTypeReferenceDirective:
266 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 ).
268 #### getCustomTransformers _( (program: Program) => { before?: TransformerFactory<SourceFile>[]; after?: TransformerFactory<SourceFile>[]; } )_
270 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 ).
272 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 ))
274 #### logInfoToStdOut _(boolean) (default=false)_
276 This is important if you read from stdout or stderr and for proper error handling.
277 The default value ensures that you can read from stdout e.g. via pipes or you use webpack -j to generate json output.
279 #### logLevel _(string) (default=warn)_
281 Can be `info` , `warn` or `error` which limits the log output to the specified log level.
282 Beware of the fact that errors are written to stderr and everything else is written to stderr (or stdout if logInfoToStdOut is true).
284 #### silent _(boolean) (default=false)_
286 If `true` , no console.log messages will be emitted. Note that most error
287 messages are emitted via webpack which is not affected by this flag.
289 #### ignoreDiagnostics _(number[]) (default=[])_
291 You can squelch certain TypeScript errors by specifying an array of diagnostic
292 codes to ignore.
294 #### reportFiles _(string[]) (default=[])_
296 Only report errors on files matching these glob patterns.
298 ```javascript
299 // in webpack.config.js
300 {
301 test: /\.ts$/,
302 loader: 'ts-loader',
303 options: { reportFiles: ['src/**/*.{ts,tsx}', '!src/skip.ts'] }
304 }
305 ```
307 This can be useful when certain types definitions have errors that are not fatal to your application.
309 #### compiler _(string) (default='typescript')_
311 Allows use of TypeScript compilers other than the official one. Should be
312 set to the NPM name of the compiler, eg [ `ntypescript` ]( https://github.com/basarat/ntypescript ).
314 #### configFile _(string) (default='tsconfig.json')_
316 Allows you to specify where to find the TypeScript configuration file.
318 You may provide
320 * 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.
321 * a relative path to the configuration file. It will be resolved relative to the respective `.ts` entry file.
322 * an absolute path to the configuration file.
324 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).
325 In this case the `configFile` should point to the `tsconfig.json` and `context` to the project root.
327 #### colors _(boolean) (default=true)_
329 If `false` , disables built-in colors in logger messages.
331 #### errorFormatter _((message: ErrorInfo, colors: boolean) => string) (default=undefined)_
333 By default ts-loader formats TypeScript compiler output for an error or a warning in the style:
335 ```
336 [tsl] ERROR in myFile.ts(3,14)
337 TS4711: you did something very wrong
338 ```
340 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.)
342 ```javascript
343 function customErrorFormatter(error, colors) {
344 const messageColor =
345 error.severity === "warning" ? colors.bold.yellow : colors.bold.red;
346 return (
347 "Does not compute.... " +
348 messageColor(Object.keys(error).map(key => `${key}: ${error[key]}`))
349 );
350 }
351 ```
353 If the above formatter received an error like this:
355 ```json
356 {
357 "code":2307,
358 "severity": "error",
359 "content": "Cannot find module 'components/myComponent2'.",
360 "file":"/.test/errorFormatter/app.ts",
361 "line":2,
362 "character":31
363 }
364 ```
366 It would produce an error message that said:
368 ```
369 Does not compute.... code: 2307,severity: error,content: Cannot find module 'components/myComponent2'.,file: /.test/errorFormatter/app.ts,line: 2,character: 31
370 ```
372 And the bit after "Does not compute.... " would be red.
374 #### compilerOptions _(object) (default={})_
376 Allows overriding TypeScript options. Should be specified in the same format
377 as you would do for the `compilerOptions` property in tsconfig.json.
379 #### instance _(string)_
381 Advanced option to force files to go through different instances of the
382 TypeScript compiler. Can be used to force segregation between different parts
383 of your code.
385 #### appendTsSuffixTo _(RegExp[]) (default=[])_
387 #### appendTsxSuffixTo _(RegExp[]) (default=[])_
389 A list of regular expressions to be matched against filename. If filename matches one of the regular expressions, a `.ts` or `.tsx` suffix will be appended to that filename.
391 This is useful for `*.vue` [ file format ]( https://vuejs.org/v2/guide/single-file-components.html ) for now. (Probably will benefit from the new single file format in the future.)
393 Example:
395 webpack.config.js:
397 ```javascript
398 module.exports = {
399 entry: "./index.vue",
400 output: { filename: "bundle.js" },
401 resolve: {
402 extensions: [".ts", ".vue"]
403 },
404 module: {
405 rules: [
406 { test: /\.vue$/, loader: "vue-loader" },
407 {
408 test: /\.ts$/,
409 loader: "ts-loader",
410 options: { appendTsSuffixTo: [/\.vue$/] }
411 }
412 ]
413 }
414 };
415 ```
417 index.vue
419 ```vue
420 <template><p>hello {{msg}}</p></template>
421 <script lang="ts">
422 export default {
423 data(): Object {
424 return {
425 msg: "world"
426 };
427 }
428 };
429 </script>
430 ```
432 We can handle `.tsx` by quite similar way:
434 webpack.config.js:
436 ```javascript
437 module.exports = {
438 entry: './index.vue',
439 output: { filename: 'bundle.js' },
440 resolve: {
441 extensions: ['.ts', '.tsx', '.vue', '.vuex']
442 },
443 module: {
444 rules: [
445 { test: /\.vue$/, loader: 'vue-loader',
446 options: {
447 loaders: {
448 ts: 'ts-loader',
449 tsx: 'babel-loader!ts-loader',
450 }
451 }
452 },
453 { test: /\.ts$/, loader: 'ts-loader', options: { appendTsSuffixTo: [/TS\.vue$/] } }
454 { test: /\.tsx$/, loader: 'babel-loader!ts-loader', options: { appendTsxSuffixTo: [/TSX\.vue$/] } }
455 ]
456 }
457 }
458 ```
460 tsconfig.json (set `jsx` option to `preserve` to let babel handle jsx)
462 ```json
463 {
464 "compilerOptions": {
465 "jsx": "preserve"
466 }
467 }
468 ```
470 index.vue
472 ```vue
473 <script lang="tsx">
474 export default {
475 functional: true,
476 render(h, c) {
477 return (<div>Content</div>);
478 }
479 }
480 </script>
481 ```
483 Or if you want to use only tsx, just use the `appendTsxSuffixTo` option only:
485 ```javascript
486 { test: /\.ts$/, loader: 'ts-loader' }
487 { test: /\.tsx$/, loader: 'babel-loader!ts-loader', options: { appendTsxSuffixTo: [/\.vue$/] } }
488 ```
490 #### onlyCompileBundledFiles _(boolean) (default=false)_
492 The default behavior of ts-loader is to act as a drop-in replacement for the `tsc` command,
493 so it respects the `include` , `files` , and `exclude` options in your `tsconfig.json` , loading
494 any files specified by those options. The `onlyCompileBundledFiles` option modifies this behavior,
495 loading only those files that are actually bundled by webpack, as well as any `.d.ts` files included
496 by the `tsconfig.json` settings. `.d.ts` files are still included because they may be needed for
497 compilation without being explicitly imported, and therefore not picked up by webpack.
499 #### allowTsInNodeModules _(boolean) (default=false)_
501 By default, ts-loader will not compile `.ts` files in `node_modules` .
502 You should not need to recompile `.ts` files there, but if you really want to, use this option.
503 Note that this option acts as a *whitelist* - any modules you desire to import must be included in
504 the `"files"` or `"include"` block of your project's `tsconfig.json` .
506 See: [ https://github.com/Microsoft/TypeScript/issues/12358 ]( https://github.com/Microsoft/TypeScript/issues/12358 )
508 ```javascript
509 // in webpack.config.js
510 {
511 test: /\.ts$/,
512 loader: 'ts-loader',
513 options: { allowTsInNodeModules: true }
514 }
515 ```
517 And in your `tsconfig.json` :
519 ```json
520 {
521 "include": [
522 "node_modules/whitelisted_module.ts"
523 ],
524 "files": [
525 "node_modules/my_module/whitelisted_file.ts"
526 ]
527 }
528 ```
530 #### context _(string) (default=undefined)_
532 If set, will parse the TypeScript configuration file with given **absolute path** as base path.
533 Per default the directory of the configuration file is used as base path. Relative paths in the configuration
534 file are resolved with respect to the base path when parsed. Option `context` allows to set option
535 `configFile` to a path other than the project root (e.g. a NPM package), while the base path for `ts-loader`
536 can remain the project root.
538 Keep in mind that **not** having a `tsconfig.json` in your project root can cause different behaviour between `ts-loader` and `tsc` .
539 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
540 referenced in option `configFile` . For more information please [ read the PR ]( https://github.com/TypeStrong/ts-loader/pull/681 ) that
541 is the base and [ read the PR ]( https://github.com/TypeStrong/ts-loader/pull/688 ) that contributed this option.
543 webpack:
545 ```javascript
546 {
547 loader: require.resolve('ts-loader'),
548 options: {
549 context: __dirname,
550 configFile: require.resolve('ts-config-react-app')
551 }
552 }
553 ```
555 Extending `tsconfig.json` :
557 ```json
558 { "extends": "./node_modules/ts-config-react-app/index" }
559 ```
561 Note that changes in the extending file while not be respected by `ts-loader` . Its purpose is to satisfy the code editor.
563 #### experimentalFileCaching _(boolean) (default=true)_
565 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).
566 In some cases it may produce performance degradation.
568 This flag enables caching for some FS-functions like `fileExists` , `realpath` and `directoryExists` for TypeScript compiler. Note that caches are cleared between compilations.
570 #### projectReferences _(boolean) (default=false)_
572 **TL;DR:** Using project references currently requires building referenced projects outside of ts-loader. We don’t want to keep it that way, but we’re releasing what we’ve got now. To try it out, you’ll need to pass `projectReferences: true` to `loaderOptions` . You’ll also probably need to use TypeScript 3.1.1 or later (which, as of this writing, means `typescript@next` ).
574 ts-loader has partial support for [ project references ]( https://www.typescriptlang.org/docs/handbook/project-references.html ) in that it will _load_ dependent composite projects that are already built, but will not currently _build/rebuild_ those upstream projects. The best way to explain exactly what this means is through an example. Say you have a project with a project reference pointing to the `lib/` directory:
576 ```
577 tsconfig.json
578 app.ts
579 lib/
580 tsconfig.json
581 niftyUtil.ts
582 ```
584 And we’ll assume that the root `tsconfig.json` has `{ "references": { "path": "lib" } }` , which means that any import of a file that’s part of the `lib` sub-project is treated as a reference to another project, not just a reference to a TypeScript file. Before discussing how ts-loader handles this, it’s helpful to review at a really basic level what `tsc` itself does here. If you were to run `tsc` on this tiny example project, the build would fail with the error:
586 ```
587 error TS6305: Output file 'lib/niftyUtil.d.ts' has not been built from source file 'lib/niftyUtil.ts'.
588 ```
590 Using project references actually instructs `tsc` _not_ to build anything that’s part of another project from source, but rather to look for any `.d.ts` and `.js` files that have already been generated from a previous build. Since we’ve never built the project in `lib` before, those files don’t exist, so building the root project fails. Still just thinking about how `tsc` works, there are two options to make the build succeed: either run `tsc -p lib/tsconfig.json` _first_ , or simply run `tsc --build` , which will figure out that `lib` hasn’t been built and build it first for you.
592 Ok, so how is that relevant to ts-loader? Because the best way to think about what ts-loader does with project references is that it acts like `tsc` , but _not_ like `tsc --build` . If you run ts-loader on a project that’s using project references, and any upstream project hasn’t been built, you’ll get the exact same `error TS6305` that you would get with `tsc` . If you modify a source file in an upstream project and don’t rebuild that project, `ts-loader` won’t have any idea that you’ve changed anything—it will still be looking at the output from the last time you _built_ that file.
594 **“Hey, don’t you think that sounds kind of useless and terrible?”** Well, sort of. You can consider it a work-in-progress. It’s true that on its own, as of today, ts-loader doesn’t have everything you need to take advantage of project references in webpack. In practice, though, _consuming_ upstream projects and _building_ upstream projects are somewhat separate concerns. Building them will likely come in a future release. For background, see the [ original issue ]( https://github.com/TypeStrong/ts-loader/issues/815 ).
596 **`outDir` Windows problemo.** At the moment, composite projects built using the [ `outDir` compiler option ]( https://www.typescriptlang.org/docs/handbook/compiler-options.html ) cannot be consumed using ts-loader on Windows. If you try to, ts-loader throws a "has not been built from source file" error. We don't know why yet; it's possible there's a bug in `tsc` . It's more likely there's a bug in `ts-loader` . Hopefully it's going to get solved at some point. (Hey, maybe you're the one to solve it!) Either way, we didn't want to hold back from releasing. So if you're building on Windows then avoid building `composite` projects using `outDir` .
598 **TypeScript version compatibility.** As a final caveat, [ this commit to TypeScript ]( https://github.com/Microsoft/TypeScript/commit/d519e3f21ec36274726c44dab25c9eb48e34953f ) is necessary for the `include` or `exclude` options of a project-referenced tsconfig file to work. It should be released in TypeScript 3.1.1 according to the tags. To use an earlier version of TypeScript, referenced project configuration files must specify `files` , and not `include` .
600 ### Usage with webpack watch
602 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:
604 ```javascript
605 plugins: [
606 new webpack.WatchIgnorePlugin([
607 /\.js$/,
608 /\.d\.ts$/
609 ])
610 ],
611 ```
613 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.
615 ### Hot Module replacement
617 To enable `webpack-dev-server` HMR, you need to follow the official [ webpack HMR guide ]( https://webpack.js.org/guides/hot-module-replacement/ ), then tweak a few config options for `ts-loader` . The required configuration is as follows:
619 1. Set `transpileOnly` to `true` (see [ transpileOnly ]( #transpileonly-boolean-defaultfalse ) for config details and recommendations above).
620 2. Inside your HMR acceptance callback function, you must re-require the module that was replaced.
622 For a boilerplate HMR project using React, check out the [ react-hot-boilerplate example ]( ./examples/react-hot-boilerplate/ ).
624 For a minimal HMR TypeScript setup, go to the [ hot-module-replacement example ]( ./examples/hot-module-replacement/ ).
626 ## Contributing