A loader is just 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.

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 just 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);
  });
};

Loaders were originally designed to work in synchronous loader pipelines, like Node.js (using enhanced-require), and asynchronous pipelines, like in webpack. However, since expensive synchronous computations are a bad idea in a single-threaded environment like Node.js, we advise making your loader asynchronous if possible. Synchronous loaders are ok if the amount of computation is trivial.

"Raw" Loader

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

Loaders may be added inline in requests and disabled via inline prefixes, which will impact the order in which they are "pitched" and executed. See Rule.enforce for more details.

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

See the bundle-loader for a good example of how this process can be used in a more meaningful way.

The Loader Context

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

Given the following example, this require call is used:

In /abc/file.js:

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

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.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.rootContext

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

this.request

The resolved request string.

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

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 ?.

Use the getOptions method from loader-utils to extract given loader options.

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).

It can be useful to pass an abstract syntax tree (AST), like ESTree, as the fourth argument (meta) to speed up the build time if you want to share common ASTs between loaders.

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

this.async

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

this.data

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

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.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.loaderIndex

The index in the loaders array of the current loader.

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

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.target

Target of compilation. Passed from configuration options.

Example values: "web", "node"

this.webpack

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

Loaders were originally designed to also work as Babel transforms. Therefore, if you write a loader that works for both, you can use this property to know if there is access to additional loaderContext and webpack features.

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.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

Note that the warnings will not be displayed if stats.warnings is set to false, or some other omit setting is used to stats such as none or errors-only. See the stats configuration.

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

Unlike throwing an Error directly, it will NOT interrupt the compilation process of the current module.

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.resolve

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

Resolve a request like a require expression.

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.addContextDependency

addContextDependency(directory: string)

Add a directory as dependency of the loader result.

this.clearDependencies

clearDependencies()

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

this.emitFile

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

Emit a file. This is webpack-specific.

this.fs

Access to the compilation's inputFileSystem property.

this.mode

Read in which mode webpack is running.

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

Deprecated context properties

The usage of these properties is highly discouraged since we are planning to remove them from the context. They are still listed here for documentation purposes.

this.exec

exec(code: string, filename: string)

Execute some code fragment like a module. See this comment for a replacement method if needed.

this.resolveSync

resolveSync(context: string, request: string) -> string

Resolve a request like a require expression.

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.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.options

The options property has been deprecated in webpack 3 and removed in webpack 4.

this.debug

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

this.minimize

Tells if result should be minimized.

this._compilation

Hacky access to the Compilation object of webpack.

this._compiler

Hacky access to the Compiler object of webpack.

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

The loader path in the error is displayed since webpack 4.12

All the errors and warnings will be recorded into stats. Please see Stats Data.

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.

It is not recommended to use this syntax in application code. Inline request syntax is intended to only be used by loader generated code. Not following this recommendation will make your code webpack-specific and non-standard.

A relative matchResource will resolve relative to the current context of the containing module.

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 stringifyRequest = require('loader-utils').stringifyRequest;
const getRemainingRequest = require('loader-utils').getRemainingRequest;
const getStylesLoader = require.resolve('./getStyle');

module.exports = function (source) {
  if (STYLES_REGEXP.test(source)) {
    source = source.replace(STYLES_REGEXP, '');
    const remReq = getRemainingRequest(this);
    return `import ${stringifyRequest(`${this.resource}.css!=!${getStylesLoader}!${remReq}`)};${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.