Loader API

Loaders are transformations that are applied on the source code of a module. They are functions (running in Node.js) that take the source of a resource file as the parameter and return the new source.

How to write a loader

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 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.


Sync Loader


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


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

Async Loader


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


module.exports = function(content) {
    var callback = this.async();
    someAsyncOperation(content, function(err, result, sourceMaps, ast) {
        if(err) return callback(err);
        callback(null, result, sourceMaps, ast);
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 to make your loader asynchronously 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.


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. But, in some cases, loaders do not care about the results of the previous loader or the resource. They only care for metadata. The pitch method on the loaders is called from left to right before the loaders are called (from right to left).

If a loader delivers a result in the pitch method the process turns around and skips the remaining loaders, continuing with the calls to the more left loaders. data can be passed between pitch and normal call.

module.exports = function(content) {
    return someSyncOperation(content, this.data.value);
module.exports.pitch = function(remainingRequest, precedingRequest, data) {
    if(someCondition()) {
        // fast exit
        return "module.exports = require(" + JSON.stringify("-!" + remainingRequest) + ");";
    data.value = 42;

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:



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.


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

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


The resolved request string.

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


  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 property is deprecated as options is replacing query. Use the getOptions method from the loader-utils to extract the given loader options.


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

    err: Error | null,
    content: string | Buffer,
    sourceMap?: SourceMap,
    abstractSyntaxTree?: AST
  1. The first argument must be an Error or null
  2. The second argument a string or a Buffer.
  3. Optional: The third argument must be a source map that is parsable by this module.
  4. Optional: AST can be an Abstract Syntax Tree of the given language, like ESTree. This value is ignored by webpack itself, but useful 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 ambigious loader results.


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


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


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 other dependencies than specified with this.addDependency.


An array of all the loaders. It is writeable 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]


The index in the loaders array of the current loader.

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


The resource part of the request, including query.

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


The resource file.

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


The query of the resource.

In the example: "?rrr"


Target of compilation. Passed from configuration options.

Example values: "web", "node"


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.


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


emitWarning(message: string)

Emit a warning.


emitError(message: string)

Emit an error.


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.


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

Resolve a request like a require expression.


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

Adds a file as dependency of the loader result in order to make them watchable. For example, html-loader uses this technique as it finds src and src-set attributes. Then, it sets the url's for those attributes as dependencies of the html file that is parsed.


addContextDependency(directory: string)

Add a directory as dependency of the loader result.



Remove all dependencies of the loader result. Even initial dependencies and these of other loaders. Consider using pitch.


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

Emit a file. This is webpack-specific.


Access to the compilation's inputFileSystem property.

Deprecated context properties

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


exec(code: string, filename: string)

Execute some code fragment like a module.


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

Resolve a request like a require expression.


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


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


The options passed to the Compiler.


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


Should the result be minimized.


Hacky access to the Compilation object of webpack.


Hacky access to the Compiler object of webpack.


Hacky access to the Module object being loaded.

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Licensed under the Creative Commons Attribution License 4.0.