/webpack 1



Loaders allow you to preprocess files as you require() or “load” them. Loaders are kind of like “tasks” in other build tools, and provide a powerful way to handle frontend build steps. Loaders can transform files from a different language like, CoffeeScript to JavaScript, or inline images as data URLs. Loaders even allow you to do things like require() css files right in your JavaScript!

To tell Webpack to transform a module with a loader, you can specify the loader in the Webpack configuration file (preferred) or in the module request, such as in a require call.

var moduleWithOneLoader = require("my-loader!./my-awesome-module");

Notice the ! syntax separating the loader from the module path? Loaders, like modules can also be specified with a relative path (as if you were requiring it) instead of the loader name:


Loaders can be also be chained together by separating loaders with the !. This is helpful for applying multiple transformations to a file in a pipeline.


When chaining loaders, they are applied right to left (from the file, back). In the above example, my-styles.less will be transformed first by the less-loaderconverting it to css, and then passed to the css-loader where urls, fonts, and other resources are processed, and then finally passed to style-loader to be transformed into a <style> tag.


Loaders can accept query parameters:


The format of the query string is up to the loader, so check the loaders documentation to find out about the parameters the loader accept, but generally most loaders support the traditional query string format.

loaders by config

Specifying loaders in each module request can be brittle and repetitive. Webpack provides a way to specify which loaders apply to different file types in your Webpack configuration file. Specifying loaders in the configuration is the recommended approach in most cases as it doesn’t add any build specific syntax to the code, making it more reusable.

    module: {
        loaders: [
            { test: /\.coffee$/, loader: "coffee-loader" }
        preLoaders: [
            { test: /\.coffee$/, loader: "coffee-hint-loader" }

See the configuration page for more information about configuring loaders.

Note that loaders such as coffee-loader or json-loader are modules, which you can install from NPM. For example:

npm install json-loader --save-dev

loader order

After the file is read from the filesystem, loaders are executed against it in the following order.

  1. preLoaders specified in the configuration
  2. loaders specified in the configuration
  3. loaders specified in the request (e.g. require('raw!./file.js'))
  4. postLoaders specified in the configuration

You can also override the configuration loader order in the module request to suit special cases.

  • adding ! to a request will disable configured preLoaders
    • require("!raw!./script.coffee")
  • adding !! to a request will disable all loaders specified in the configuration
    • require("!!raw!./script.coffee")
  • adding -! to a request will disable configured preLoaders and loaders but not the postLoaders
    • require("-!raw!./script.coffee")


It is recommended that the result is JavaScript after step 2.

It is recommended to apply non-JavaScript to JavaScript transformations in step 1 (or step 2 when they don’t apply globally).

It is recommended to stay in the same language in pre and post loaders.

Source code that want to override the non-js to js transformation should use the ! prefix. (i. e. to transform it in another way)

Using the !! and -! prefix to disable loaders is not recommended except from another loader.

  • Example for a preLoader: Image compression
  • Example for a loader (in config): coffee-script transformation
  • Example for a loader (in request): bundle loader
  • Example for a postLoader: Code coverage instrumenting

Writing a loader

Writing a loader is pretty simple. A loader is just a file that exports a function. The compiler 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 the compiler with some useful methods that allow the loader to, among other things, 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. It returns this.callback if async mode is allowed. Then the loader must return undefined and call the callback.

Errors can be thrown in sync mode or the this.callback can be called with the error.

webpack allows async mode in every case.

enhanced-require allows async mode only with require.ensure or AMD require.

For more detailed instructions and guidelines, check out How to write a loader.


sync loader

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

async loader

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

Note: It’s recommended to give an asynchronous loader a fall back to synchronous mode. This isn’t required for webpack, but allows to run the loader sync using enhanced-require.

raw loader

By default the resource file is treated as utf-8 string and passed as String to the loader. By setting raw to true the loader is passed 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

The loaders are 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. 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;

loader context

This stuff is available on this in a loader.

For the example this require call is used:

In /abc/file.js:



Loader API version. Currently 1.


A string. 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"


A string. The query of the request for the current loader.

In the example: in loader1: "?xyz", in loader2: ""


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


cacheable(flag = true: boolean)

Make this loader result cacheable. By default it’s 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. Most loaders are deterministic and cacheable.


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

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

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"


emitWarning(message: string)

Emit a warning.


emitError(message: string)

Emit an error.


exec(code: string, filename: string)

Execute some code fragment like a module.

Hint: Don’t use require(this.resourcePath), use this function to make loaders chainable!


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

Resolve a request like a require expression.


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

Resolve a request like a require expression.


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

Add a file as dependency of the loader result in order to make them watchable.


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.


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.


Should a SourceMap be generated.


Target of compilation. Passed from configuration options.

Example values: "web", "node"


Set to true when this is compiled by webpack.


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

Emit a file. This is webpack-specific


Hacky access to the Compilation object of webpack.


Hacky access to the Compiler object of webpack.

© 2012–2015 Tobias Koppers
Licensed under the MIT License.