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What are loaders?

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

For example, you can use loaders to tell webpack to load CoffeeScript or JSX.

Loader features

  • Loaders can be chained. They are applied in a pipeline to the resource. The final loader is expected to return JavaScript, the other can return arbitrary format (which is passed to the next loader)
  • Loaders can be synchronous and asynchronous.
  • Loaders run in node.js and can do everything that’s possible there.
  • Loaders accept query parameters. This can be used to pass configuration to the loader.
  • Loaders can be bound to extension / RegExps in the configuration.
  • Loaders can be published / installed through npm.
  • Normal modules can export a loader in addition to the normal main via package.json loader.
  • Loaders can access the configuration.
  • Plugins can give loaders more features.
  • Loaders can emit additional arbitrary files.
  • etc.

If you are interested in some loader examples head of to the list of loaders.

Resolving loaders

Loaders are resolved similar to modules. A loader module is expected to export a function and to be written in node.js compatible JavaScript. In the common case you manage loaders with npm, but you can also have loaders as files in your app.

Referencing loaders

By convention, though not required, loaders are usually named as XXX-loader, where XXX is the context name. For example, json-loader.

You may reference loaders by its full (actual) name (e.g. json-loader), or by its shorthand name (e.g. json).

The loader name convention and precedence search order is defined by resolveLoader.moduleTemplates within the webpack configuration API.

Loader name conventions may be useful, especially when referencing them within require() statements; see usage below.

Installing loaders

If the loader is available on npm you can install the loader via:

$ npm install xxx-loader --save

or

$ npm install xxx-loader --save-dev

Usage

There are multiple ways to use loaders in your app:

  • explicit in the require statement
  • configured via configuration
  • configured via CLI

loaders in require

Note: Avoid using this, if at all possible, if you intend your scripts to be environment agnostic (node.js and browser). Use the configuration convention for specifying loaders (see next section).

It’s possible to specify the loaders in the require statement (or define, require.ensure, etc.). Just separate loaders from resource with !. Each part is resolved relative to the current directory.

require("./loader!./dir/file.txt");
// => uses the file "loader.js" in the current directory to transform
//    "file.txt" in the folder "dir".

require("jade!./template.jade");
// => uses the "jade-loader" (that is installed from npm to "node_modules")
//    to transform the file "template.jade"

require("!style!css!less!bootstrap/less/bootstrap.less");
// => the file "bootstrap.less" in the folder "less" in the "bootstrap"
//    module (that is installed from github to "node_modules") is
//    transformed by the "less-loader". The result is transformed by the
//    "css-loader" and then by the "style-loader".

Configuration

You can bind loaders to a RegExp via configuration:

{
    module: {
        loaders: [
            { test: /\.jade$/, loader: "jade" },
            // => "jade" loader is used for ".jade" files

            { test: /\.css$/, loader: "style!css" },
            // => "style" and "css" loader is used for ".css" files
            // Alternative syntax:
            { test: /\.css$/, loaders: ["style", "css"] },
        ]
    }
}

CLI

You can bind loaders to an extension via CLI:

$ webpack --module-bind jade --module-bind 'css=style!css'

This uses the loader “jade” for “.jade” files and the loaders “style” and “css” for “.css” files.

Query parameters

Loader can be passed query parameters via a query string (just like in the web). The query string is appended to the loader with ?. i.e. url-loader?mimetype=image/png.

Note: The format of the query string is up to the loader. See format in the loader documentation. Most loaders accept parameters in the normal query format (?key=value&key2=value2) and as JSON object (?{"key":"value","key2":"value2"}).

in require

require("url-loader?mimetype=image/png!./file.png");

Configuration

{ test: /\.png$/, loader: "url-loader?mimetype=image/png" }

or

{
    test: /\.png$/,
    loader: "url-loader",
    query: { mimetype: "image/png" }
}

CLI

webpack --module-bind "png=url-loader?mimetype=image/png"
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