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Loaders

Loaders are transformations that are applied to the source code of a module. They allow you to pre-process files as you import or “load” them. Thus, loaders are kind of like “tasks” in other build tools and provide a powerful way to handle front-end build steps. Loaders can transform files from a different language (like TypeScript) to JavaScript or load inline images as data URLs. Loaders even allow you to do things like import CSS files directly from your JavaScript modules!

Example

For example, you can use loaders to tell webpack to load a CSS file or to convert TypeScript to JavaScript. To do this, you would start by installing the loaders you need:

npm install --save-dev css-loader ts-loader

And then instruct webpack to use the css-loader for every .css file and the ts-loader for all .ts files:

webpack.config.js

module.exports = {
  module: {
    rules: [
      { test: /\.css$/, use: 'css-loader' },
      { test: /\.ts$/, use: 'ts-loader' }
    ]
  }
};

Using Loaders

There are three ways to use loaders in your application:

Configuration (recommended): Specify them in your webpack.config.js file.
Inline: Specify them explicitly in each import statement.
CLI: Specify them within a shell command.

Configuration

module.rules allows you to specify several loaders within your webpack configuration. This is a concise way to display loaders, and helps to maintain clean code. It also offers you a full overview of each respective loader.

Loaders are evaluated/executed from right to left (or from bottom to top). In the example below execution starts with sass-loader, continues with css-loader and finally ends with style-loader. See "Loader Features" for more information about loaders order.

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/,
        use: [
          // style-loader
          { loader: 'style-loader' },
          // css-loader
          {
            loader: 'css-loader',
            options: {
              modules: true
            }
          },
          // sass-loader
          { loader: 'sass-loader' }
        ]
      }
    ]
  }
};

Inline

It's possible to specify loaders in an import statement, or any equivalent "importing" method. Separate loaders from the resource with !. Each part is resolved relative to the current directory.

import Styles from 'style-loader!css-loader?modules!./styles.css';

It's possible to override any loaders, preLoaders and postLoaders from the configuration by prefixing the inline import statement:

Prefixing with ! will disable all configured normal loaders

import Styles from '!style-loader!css-loader?modules!./styles.css';

Prefixing with !! will disable all configured loaders (preLoaders, loaders, postLoaders)

import Styles from '!!style-loader!css-loader?modules!./styles.css';

Prefixing with -! will disable all configured preLoaders and loaders but not postLoaders

import Styles from '-!style-loader!css-loader?modules!./styles.css';

Options can be passed with a query parameter, e.g. ?key=value&foo=bar, or a JSON object, e.g. ?{"key":"value","foo":"bar"}.

Use module.rules whenever possible, as this will reduce boilerplate in your source code and allow you to debug or locate a loader faster if something goes south.

CLI

You can also use loaders through the CLI:

webpack --module-bind pug-loader --module-bind 'css=style-loader!css-loader'

This uses the pug-loader for .jade files, and the style-loader and css-loader for .css files.

Loader Features

Loaders can be chained. Each loader in the chain applies transformations to the processed resource. A chain is executed in reverse order. The first loader passes its result (resource with applied transformations) to the next one, and so forth. Finally, webpack expects JavaScript to be returned by the last loader in the chain.
Loaders can be synchronous or asynchronous.
Loaders run in Node.js and can do everything that’s possible there.
Loaders can be configured with an options object (using query parameters to set options is still supported but has been deprecated).
Normal modules can export a loader in addition to the normal main via package.json with the loader field.
Plugins can give loaders more features.
Loaders can emit additional arbitrary files.

Loaders provide a way to customize the output through their preprocessing functions. Users now have more flexibility to include fine-grained logic such as compression, packaging, language translations and more.

Resolving Loaders

Loaders follow the standard module resolution. In most cases it will be loaded from the module path (think npm install, node_modules).

A loader module is expected to export a function and be written in Node.js compatible JavaScript. They are most commonly managed with npm, but you can also have custom loaders as files within your application. By convention, loaders are usually named xxx-loader (e.g. json-loader). See "Writing a Loader" for more information.

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