Plugin API

Plugins are a key piece of the webpack ecosystem and provide the community with a powerful way to tap into webpack's compilation process. A plugin is able to hook into key events that are fired throughout each compilation. Every step of the way, the plugin will have full access to the compiler and, when applicable, the current compilation.

Let's start by going over tapable utility, which provides the backbone of webpack's plugin interface.

Tapable

This small library is a core utility in webpack but can also be used elsewhere to provide a similar plugin interface. Many objects in webpack extend the Tapable class. The class exposes tap, tapAsync, and tapPromise methods which plugins can use to inject custom build steps that will be fired throughout a compilation.

Please see the documentation to learn more. An understanding of the three tap methods, as well as the hooks that provide them, is crucial. The objects that extend Tapable (e.g. the compiler), the hooks they provide, and each hook's type (e.g. the SyncHook) will be noted.

Plugin Types

Depending on the hooks used and tap methods applied, plugins can function in a different number of ways. The way this works is closely related to the hooks provided by Tapable. The compiler hooks each note the underlying Tapable hook indicating which tap methods are available.

So depending on which event you tap into, the plugin may run differently. For example, when hooking into the compile stage, only the synchronous tap method can be used:

compiler.hooks.compile.tap('MyPlugin', (params) => {
  console.log('Synchronously tapping the compile hook.');
});

However, for run which utilizes the AsyncHook, we can utilize tapAsync or tapPromise (as well as tap):

compiler.hooks.run.tapAsync(
  'MyPlugin',
  (source, target, routesList, callback) => {
    console.log('Asynchronously tapping the run hook.');
    callback();
  }
);

compiler.hooks.run.tapPromise('MyPlugin', (source, target, routesList) => {
  return new Promise((resolve) => setTimeout(resolve, 1000)).then(() => {
    console.log('Asynchronously tapping the run hook with a delay.');
  });
});

compiler.hooks.run.tapPromise(
  'MyPlugin',
  async (source, target, routesList) => {
    await new Promise((resolve) => setTimeout(resolve, 1000));
    console.log('Asynchronously tapping the run hook with a delay.');
  }
);

The moral of the story is that there are a variety of ways to hook into the compiler, each one allowing your plugin to run as it sees fit.

Custom Hooks

In order to offer a custom hook to the compilation for other plugins to tap into, you need to do the following:

1. Create a module-scope WeakMap for compilation hooks:

   const compilationHooks = new WeakMap<Compilation, MyHooks>();
   
   interface MyHooks {
     custom: SyncHook<[number, string]>;
   }

2. Create a static method on your plugin:

   static getCompilationHooks(compilation: Compilation) : MyHooks {
     let hooks = compilationHooks.get(compilation);
     if(hooks === undefined) {
       compilationHooks.set(compilation, hooks = {
         custom: new SyncHook()
       });
     }
     return hooks;
   }

3. Call hooks like below in your plugin:

   const hooks = MyPlugin.getCompilationHooks(compilation);
   
   hooks.custom.call(1, 'hello');

4. Other plugins can access your custom hooks too:

   import MyPlugin from 'my-plugin';
   
   const hooks = MyPlugin.getCompilationHooks(compilation);
   
   hooks.custom.tap('OtherPlugin', (n, s) => {
     // magic
   });

Again, see the documentation for tapable to learn more about the different hook classes and how they work.

Reporting Progress

Plugins can report progress via ProgressPlugin, which prints progress messages to stderr by default. In order to enable progress reporting, pass a --progress argument when running the webpack CLI.

It is possible to customize the printed output by passing different arguments to the reportProgress function of ProgressPlugin.

To report progress, a plugin must tap into a hook using the context: true option:

compiler.hooks.emit.tapAsync(
  {
    name: 'MyPlugin',
    context: true,
  },
  (context, compiler, callback) => {
    const reportProgress = context && context.reportProgress;
    if (reportProgress) reportProgress(0.95, 'Starting work');
    setTimeout(() => {
      if (reportProgress) reportProgress(0.95, 'Done work');
      callback();
    }, 1000);
  }
);

The reportProgress function may be called with these arguments:

reportProgress(percentage, ...args);

• percentage: This argument is unused; instead, ProgressPlugin will calculate a percentage based on the current hook.
• ...args: Any number of strings, which will be passed to the ProgressPlugin handler to be reported to the user.

Note that only a subset of compiler and compilation hooks support the reportProgress function. See ProgressPlugin for a full list.

Logging

Logging API is available since the release of webpack 4.37. When logging is enabled in stats configuration and/or when infrastructure logging is enabled, plugins may log messages which will be printed out in the respective logger format (stats, infrastructure).

• Plugins should prefer to use compilation.getLogger('PluginName') for logging. This kind of logging is stored in the Stats and formatted accordingly. It can be filtered and exported by the user.
• Plugins may use the compiler.getInfrastructureLogger('PluginName') for logging. Using infrastructure logging is not stored in the Stats and therefore not formatted. It's usually logged to the console/dashboard/GUI directly. It can be filtered by the user.
• Plugins may use special fallback logic for detecting logging support compilation.getLogger ? compilation.getLogger('PluginName') : console to provide a fallback for cases when an older webpack version is used which does not support getLogger method on compilation object.

Next Steps

See the compiler hooks section for a detailed listing of all the available compiler hooks and the parameters they make available.