Webpack loaders are used to process non-JavaScript files so that they can be packaged by Webpack. 1. Webpack only supports JS and JSON by default, and needs to process CSS, images, TypeScript and other resources through loaders; 2. The execution order of loaders is from right to left, such as eslint first and then babel; 3. Common combinations such as style-loader css-loader is used to process CSS, and file-loader is used to image resources; 4. To add loaders, you need to configure test, use and other properties in module.rules of webpack.config.js, and it is recommended to exclude node_modules to improve construction efficiency.

Webpack loaders are a very critical part of Webpack. Its function is to tell Webpack how to deal with non-JavaScript files, such as CSS, images, TypeScript, or JSX. Simply put, loader is used to "translate" these files so that they can be added to the packaging process.

1. Basic concepts of Webpack loader

Webpack can only understand JavaScript and JSON files by default. If you introduce other types of files into your project, such as .css , .scss , .png , or .ts , you need to configure the corresponding loader to handle them.

A loader can convert these files into modules that Webpack recognizes. For example:

• css-loader : Reads the CSS file and parses @import and url() in it.
• style-loader : Insert CSS into the <style></style> tag in the page.
• babel-loader : Used with Babel, translate ES6 code into ES5 with better compatibility.

2. The execution order of loader is from right to left

This is the easiest point to ignore when using Webpack loader. When you configure multiple loaders for a file type, their execution order is from right to left, or from bottom to top.

For example:

 {
  test: /\.js$/,
  use: ['babel-loader', 'eslint-loader']
}

This configuration means to check the code specifications with eslint-loader first, and then convert the JS code through babel-loader .

Therefore, when writing loader configuration, you should pay attention to the order and ensure that the logic is reasonable.

3. Common loader combination usage

In actual development, we often need to combine multiple loaders to complete tasks. For example, loading and processing CSS files are usually configured like this:

 {
  test: /\.css$/,
  use: ['style-loader', 'css-loader']
}

This combination means:

• First use css-loader to parse the content of the CSS file.
• Then use style-loader to insert the parsed style into the HTML page.

Another common example is processing image resources:

 {
  test: /\.(png|jpe?g|gif|svg)$/i,
  use: [
    {
      loader: 'file-loader',
      options: {
        name: 'images/[name].[hash:8].[ext]'
      }
    }
  ]
}

After this configuration, the image file will be copied to the output directory and will be naming according to the hash to prevent caching problems.

4. How to add loader to Webpack configuration

To add a loader, you need to modify the Webpack configuration file (usually webpack.config.js ) and add a new rule object in the module.rules array.

A typical loader rule structure is as follows:

 module: {
  rules: [
    {
      test: /\.extension$/,
      use: [
        // Put the loader name or object here],
      exclude: /node_modules/
    }
  ]
}

A few suggestions:

• test attribute is used to match the file extension.
• use indicates which loaders to use to process such files.
• Adding exclude: /node_modules/ can avoid processing third-party libraries and improve construction speed.
• If you only need a loader, you can also write loader: 'xxx-loader' directly, without array form.

Basically that's it. Webpack loader is not complicated, but it is an indispensable part of building modern front-end projects. As long as you understand its working methods and common usage, you can flexibly respond to various resource processing needs.

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