Loader Interface

A loader is a JavaScript module that exports a function. The loader runner calls this function and passes the result of the previous loader or the resource file into it. The this context of the function is filled-in by webpack and the loader runner with some useful methods that allow the loader (among other things) to change its invocation style to async, or get query parameters.

The first loader is passed one argument: the content of the resource file. The compiler expects a result from the last loader. The result should be a String or a Buffer (which is converted to a string), representing the JavaScript source code of the module. An optional SourceMap result (as a JSON object) may also be passed.

A single result can be returned in sync mode. For multiple results the this.callback() must be called. In async mode this.async() must be called to indicate that the loader runner should wait for an asynchronous result. It returns this.callback(). Then the loader must return undefined and call that callback.

/**
 *
 * @param {string|Buffer} content Content of the resource file
 * @param {object} [map] SourceMap data consumable by https://github.com/mozilla/source-map
 * @param {any} [meta] Meta data, could be anything
 */
function webpackLoader(content, map, meta) {
  // code of your webpack loader
}

Examples

The following sections provide some basic examples of the different types of loaders. Note that the map and meta parameters are optional, see this.callback below.

Synchronous Loaders

Either return or this.callback can be used to return the transformed content synchronously:

sync-loader.js

module.exports = function (content, map, meta) {
  return someSyncOperation(content);
};

The this.callback method is more flexible as it allows multiple arguments to be passed as opposed to only the content.

sync-loader-with-multiple-results.js

module.exports = function (content, map, meta) {
  this.callback(null, someSyncOperation(content), map, meta);
  return; // always return undefined when calling callback()
};

Asynchronous Loaders

For asynchronous loaders, this.async is used to retrieve the callback function:

async-loader.js

module.exports = function (content, map, meta) {
  var callback = this.async();
  someAsyncOperation(content, function (err, result) {
    if (err) return callback(err);
    callback(null, result, map, meta);
  });
};

async-loader-with-multiple-results.js

module.exports = function (content, map, meta) {
  var callback = this.async();
  someAsyncOperation(content, function (err, result, sourceMaps, meta) {
    if (err) return callback(err);
    callback(null, result, sourceMaps, meta);
  });
};

"Raw" Loader

By default, the resource file is converted to a UTF-8 string and passed to the loader. By setting the raw flag to true, the loader will receive the raw Buffer. Every loader is allowed to deliver its result as a String or as a Buffer. The compiler converts them between loaders.

raw-loader.js

module.exports = function (content) {
  assert(content instanceof Buffer);
  return someSyncOperation(content);
  // return value can be a `Buffer` too
  // This is also allowed if loader is not "raw"
};
module.exports.raw = true;

Pitching Loader

Loaders are always called from right to left. There are some instances where the loader only cares about the metadata behind a request and can ignore the results of the previous loader. The pitch method on loaders is called from left to right before the loaders are actually executed (from right to left).

For the following configuration of use:

module.exports = {
  //...
  module: {
    rules: [
      {
        //...
        use: ['a-loader', 'b-loader', 'c-loader'],
      },
    ],
  },
};

These steps would occur:

|- a-loader `pitch`
  |- b-loader `pitch`
    |- c-loader `pitch`
      |- requested module is picked up as a dependency
    |- c-loader normal execution
  |- b-loader normal execution
|- a-loader normal execution

So why might a loader take advantage of the "pitching" phase?

First, the data passed to the pitch method is exposed in the execution phase as well under this.data and could be useful for capturing and sharing information from earlier in the cycle.

module.exports = function (content) {
  return someSyncOperation(content, this.data.value);
};

module.exports.pitch = function (remainingRequest, precedingRequest, data) {
  data.value = 42;
};

Second, if a loader delivers a result in the pitch method, the process turns around and skips the remaining loaders. In our example above, if the b-loaders pitch method returned something:

module.exports = function (content) {
  return someSyncOperation(content);
};

module.exports.pitch = function (remainingRequest, precedingRequest, data) {
  if (someCondition()) {
    return (
      'module.exports = require(' +
      JSON.stringify('-!' + remainingRequest) +
      ');'
    );
  }
};

The steps above would be shortened to:

|- a-loader `pitch`
  |- b-loader `pitch` returns a module
|- a-loader normal execution

The Loader Context

The loader context represents the properties that are available inside of a loader assigned to the this property.

Example for the loader context

Given the following example, this require call is used:

In /abc/file.js:

require('./loader1?xyz!loader2!./resource?rrr');

this.addContextDependency

addContextDependency(directory: string)

Add a directory as dependency of the loader result.

this.addDependency

addDependency(file: string)
dependency(file: string) // shortcut

Add a file as dependency of the loader result in order to make them watchable. For example, sass-loader, less-loader uses this to recompile whenever any imported css file changes.

this.async

Tells the loader-runner that the loader intends to call back asynchronously. Returns this.callback.

this.cacheable

A function that sets the cacheable flag:

cacheable(flag = true: boolean)

By default, loader results are flagged as cacheable. Call this method passing false to make the loader's result not cacheable.

A cacheable loader must have a deterministic result when inputs and dependencies haven't changed. This means the loader shouldn't have dependencies other than those specified with this.addDependency.

this.callback

A function that can be called synchronously or asynchronously in order to return multiple results. The expected arguments are:

this.callback(
  err: Error | null,
  content: string | Buffer,
  sourceMap?: SourceMap,
  meta?: any
);

1. The first argument must be an Error or null
2. The second argument is a string or a Buffer.
3. Optional: The third argument must be a source map that is parsable by this module.
4. Optional: The fourth option, ignored by webpack, can be anything (e.g. some metadata).

In case this function is called, you should return undefined to avoid ambiguous loader results.

this.clearDependencies

clearDependencies();

Remove all dependencies of the loader result, even initial dependencies and those of other loaders. Consider using pitch.

this.context

The directory of the module. Can be used as a context for resolving other stuff.

In the example: /abc because resource.js is in this directory

this.data

A data object shared between the pitch and the normal phase.

this.emitError

emitError(error: Error)

Emit an error that also can be displayed in the output.

ERROR in ./src/lib.js (./src/loader.js!./src/lib.js)
Module Error (from ./src/loader.js):
Here is an Error!
 @ ./src/index.js 1:0-25

this.emitFile

emitFile(name: string, content: Buffer|string, sourceMap: {...})

Emit a file. This is webpack-specific.

this.emitWarning

emitWarning(warning: Error)

Emit a warning that will be displayed in the output like the following:

WARNING in ./src/lib.js (./src/loader.js!./src/lib.js)
Module Warning (from ./src/loader.js):
Here is a Warning!
 @ ./src/index.js 1:0-25

this.fs

Access to the compilation's inputFileSystem property.

this.getOptions(schema)

Extracts given loader options. Optionally, accepts JSON schema as an argument.

this.getResolve

getResolve(options: ResolveOptions): resolve

resolve(context: string, request: string, callback: function(err, result: string))
resolve(context: string, request: string): Promise<string>

Creates a resolve function similar to this.resolve.

Any options under webpack resolve options are possible. They are merged with the configured resolve options. Note that "..." can be used in arrays to extend the value from resolve options, e.g. { extensions: [".sass", "..."] }.

options.dependencyType is an additional option. It allows us to specify the type of dependency, which is used to resolve byDependency from the resolve options.

All dependencies of the resolving operation are automatically added as dependencies to the current module.

this.hot

Information about HMR for loaders.

module.exports = function (source) {
  console.log(this.hot); // true if HMR is enabled via --hot flag or webpack configuration
  return source;
};

this.importModule

this.importModule(request, options, [callback]): Promise

An alternative lightweight solution for the child compiler to compile and execute a request at build time.

• request: the request string to load the module from
• options:
  • layer: specify a layer in which this module is placed/compiled
  • publicPath: the public path used for the built modules
• callback: an optional Node.js style callback returning the exports of the module or a namespace object for ESM. importModule will return a Promise if no callback is provided.

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /stylesheet\.js$/i,
        use: ['./a-pitching-loader.js'],
        type: 'asset/source', // we set type to 'asset/source' as the loader will return a string
      },
    ],
  },
};

a-pitching-loader.js

exports.pitch = async function (remaining) {
  const result = await this.importModule(
    this.resourcePath + '.webpack[javascript/auto]' + '!=!' + remaining
  );
  return result.default || result;
};

src/stylesheet.js

import { green, red } from './colors.js';
export default `body { background: ${red}; color: ${green}; }`;

src/colors.js

export const red = '#f00';
export const green = '#0f0';

src/index.js

import stylesheet from './stylesheet.js';
// stylesheet will be a string `body { background: #f00; color: #0f0; }` at build time

You might notice something in the above example:

1. We have a pitching loader,
2. We use !=! syntax in that pitching loader to set matchResource for the request, i.e., we'll use this.resourcePath + '.webpack[javascript/auto]' to match with the module.rules instead of the original resource,
3. .webpack[javascript/auto] is a pseudo extension of the .webpack[type] pattern, we use it to specify a default module type when no other module type is specified. It's typically used in conjunction with !=! syntax.

Note that the above example is a simplified one, you can check the full example on webpack repository.

this.loaderIndex

The index in the loaders array of the current loader.

In the example: in loader1: 0, in loader2: 1

this.loadModule

loadModule(request: string, callback: function(err, source, sourceMap, module))

Resolves the given request to a module, applies all configured loaders and calls back with the generated source, the sourceMap and the module instance (usually an instance of NormalModule). Use this function if you need to know the source code of another module to generate the result.

this.loadModule in a loader context uses CommonJS resolve rules by default. Use this.getResolve with an appropriate dependencyType, e.g. 'esm', 'commonjs' or a custom one before using a different semantic.

this.loaders

An array of all the loaders. It is writable in the pitch phase.

loaders = [{request: string, path: string, query: string, module: function}]

In the example:

[
  {
    request: '/abc/loader1.js?xyz',
    path: '/abc/loader1.js',
    query: '?xyz',
    module: [Function],
  },
  {
    request: '/abc/node_modules/loader2/index.js',
    path: '/abc/node_modules/loader2/index.js',
    query: '',
    module: [Function],
  },
];

this.mode

Read in which mode webpack is running.

Possible values: 'production', 'development', 'none'

this.query

1. If the loader was configured with an options object, this will point to that object.
2. If the loader has no options, but was invoked with a query string, this will be a string starting with ?.

this.request

The resolved request string.

In the example: '/abc/loader1.js?xyz!/abc/node_modules/loader2/index.js!/abc/resource.js?rrr'

this.resolve

resolve(context: string, request: string, callback: function(err, result: string))

Resolve a request like a require expression.

• context must be an absolute path to a directory. This directory is used as the starting location for the resolving.
• request is the request to be resolved. Usually either relative requests like ./relative or module requests like module/path are used, but absolute paths like /some/path are also possible as requests.
• callback is a normal Node.js-style callback function giving the resolved path.

All dependencies of the resolving operation are automatically added as dependencies to the current module.

this.resource

The resource part of the request, including query.

In the example: '/abc/resource.js?rrr'

this.resourcePath

The resource file.

In the example: '/abc/resource.js'

this.resourceQuery

The query of the resource.

In the example: '?rrr'

this.rootContext

Since webpack 4, the formerly this.options.context is provided as this.rootContext.

this.sourceMap

Tells if source map should be generated. Since generating source maps can be an expensive task, you should check if source maps are actually requested.

this.target

Target of compilation. Passed from configuration options.

Example values: 'web', 'node'

this.utils

Access to contextify and absolutify utilities.

• contextify: Return a new request string avoiding absolute paths when possible.
• absolutify: Return a new request string using absolute paths when possible.

my-sync-loader.js

module.exports = function (content) {
  this.utils.contextify(
    this.context,
    this.utils.absolutify(this.context, './index.js')
  );
  this.utils.absolutify(this.context, this.resourcePath);
  // …
  return content;
};

this.version

Loader API version. Currently 2. This is useful for providing backwards compatibility. Using the version you can specify custom logic or fallbacks for breaking changes.

this.webpack

This boolean is set to true when this is compiled by webpack.

Webpack specific properties

The loader interface provides all module relate information. However in rare cases you might need access to the compiler api itself.

Therefore you should only use them as a last resort. Using them will reduce the portability of your loader.

this._compilation

Access to the current Compilation object of webpack.

this._compiler

Access to the current Compiler object of webpack.

Deprecated context properties

this.debug

A boolean flag. It is set when in debug mode.

this.inputValue

Passed from the last loader. If you would execute the input argument as a module, consider reading this variable for a shortcut (for performance).

this.minimize

Tells if result should be minimized.

this.value

Pass values to the next loader. If you know what your result exports if executed as a module, set this value here (as an only element array).

this._module

Hacky access to the Module object being loaded.

Error Reporting

You can report errors from inside a loader by:

• Using this.emitError. Will report the errors without interrupting module's compilation.
• Using throw (or other uncaught exception). Throwing an error while a loader is running will cause current module compilation failure.
• Using callback (in async mode). Pass an error to the callback will also cause module compilation failure.

For example:

./src/index.js

require('./loader!./lib');

Throwing an error from loader:

./src/loader.js

module.exports = function (source) {
  throw new Error('This is a Fatal Error!');
};

Or pass an error to the callback in async mode:

./src/loader.js

module.exports = function (source) {
  const callback = this.async();
  //...
  callback(new Error('This is a Fatal Error!'), source);
};

The module will get bundled like this:

/***/ "./src/loader.js!./src/lib.js":
/*!************************************!*\
  !*** ./src/loader.js!./src/lib.js ***!
  \************************************/
/*! no static exports found */
/***/ (function(module, exports) {

throw new Error("Module build failed (from ./src/loader.js):\nError: This is a Fatal Error!\n    at Object.module.exports (/workspace/src/loader.js:3:9)");

/***/ })

Then the build output will also display the error (Similar to this.emitError):

ERROR in ./src/lib.js (./src/loader.js!./src/lib.js)
Module build failed (from ./src/loader.js):
Error: This is a Fatal Error!
    at Object.module.exports (/workspace/src/loader.js:2:9)
 @ ./src/index.js 1:0-25

As you can see below, not only error message, but also details about which loader and module are involved:

• the module path: ERROR in ./src/lib.js
• the request string: (./src/loader.js!./src/lib.js)
• the loader path: (from ./src/loader.js)
• the caller path: @ ./src/index.js 1:0-25

Inline matchResource

A new inline request syntax was introduced in webpack v4. Prefixing <match-resource>!=! to a request will set the matchResource for this request.

When a matchResource is set, it will be used to match with the module.rules instead of the original resource. This can be useful if further loaders should be applied to the resource, or if the module type needs to be changed. It's also displayed in the stats and used for matching Rule.issuer and test in splitChunks.

Example:

file.js

/* STYLE: body { background: red; } */
console.log('yep');

A loader could transform the file into the following file and use the matchResource to apply the user-specified CSS processing rules:

file.js (transformed by loader)

import './file.js.css!=!extract-style-loader/getStyles!./file.js';
console.log('yep');

This will add a dependency to extract-style-loader/getStyles!./file.js and treat the result as file.js.css. Because module.rules has a rule matching /\.css$/ and it will apply to this dependency.

The loader could look like this:

extract-style-loader/index.js

const getStylesLoader = require.resolve('./getStyle');

module.exports = function (source) {
  if (STYLES_REGEXP.test(source)) {
    source = source.replace(STYLES_REGEXP, '');
    return `import ${JSON.stringify(
      this.utils.contextify(this.context || this.rootContext, `${this.resource}.css!=!${getStylesLoader}!${this.remainingRequest}`)
    )};${source}`;
  }
  return source;
};

extract-style-loader/getStyles.js

module.exports = function (source) {
  const match = STYLES_REGEXP.match(source);
  return match[0];
};

Logging

Logging API is available since the release of webpack 4.37. When logging is enabled in stats configuration and/or when infrastructure logging is enabled, loaders may log messages which will be printed out in the respective logger format (stats, infrastructure).

• Loaders should prefer to use this.getLogger() for logging which is a shortcut to compilation.getLogger() with loader path and processed file. This kind of logging is stored to the Stats and formatted accordingly. It can be filtered and exported by the webpack user.
• Loaders may use this.getLogger('name') to get an independent logger with a child name. Loader path and processed file is still added.
• Loaders may use special fallback logic for detecting logging support this.getLogger() ? this.getLogger() : console to provide a fallback when an older webpack version is used which does not support getLogger method.

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