Compiler

The Compiler module of webpack is the main engine that creates a compilation instance with all the options passed through webpack CLI or webpack api or webpack configuration file.

It is exported by webpack api under webpack.Compiler.

The compiler is used by webpack by instantiating it and then calling the run method. Below is a trivial example of how one might use the Compiler. In fact, this is really close to how webpack itself uses it.

compiler-example

// Can be imported from webpack package
import {Compiler} from 'webpack';

// Create a new compiler instance
const compiler = new Compiler();

// Populate all required options
compiler.options = {...};

// Creating a plugin.
class LogPlugin {
  apply (compiler) {
    compiler.plugin('should-emit', compilation => {
      console.log('should I emit?');
      return true;
    })
  }
}

// Apply the compiler to the plugin
new LogPlugin().apply(compiler);

/* Add other supporting plugins */

// Callback to be executed after run is complete
const callback = (err, stats) => {
  console.log('Compiler has finished execution.');
  // Display stats...
};

// call run on the compiler along with the callback
compiler.run(callback);

The Compiler is what we call a Tapable instance. By this, we mean that it mixes in Tapable class to imbibe functionality to register and call plugins on itself. Most user facing plugins are first registered on the Compiler. The working of a Compiler can be condensed into the following highlights

• Usually there is one master instance of Compiler. Child compilers can be created for delegating specific tasks.
• A lot of the complexity in creating a compiler goes into populating all the relevant options for it.
• webpack has WebpackOptionsDefaulter and WebpackOptionsApply specifically designed to provide the Compiler with all the initial data it requires.
• The Compiler is ultimately just a function which performs bare minimum functionality to keep a lifecycle running. It delegates all the loading/bundling/writing work to various plugins.
• new LogPlugin(args).apply(compiler) registers the plugin to any particular hook event in the Compiler's lifecycle.
• The Compiler exposes a run method which kickstarts all compilation work for webpack. When that is done, it should call the passed in callback function. All the tail end work of logging stats and errors are done in this callback function.

Watching

The Compiler supports "watch mode" which monitors the file system and recompiles as files change. When in watch mode, the compiler will emit the additional events "watch-run", "watch-close", and "invalid". This is typically used in development, usually under the hood of tools like webpack-dev-server, so that the developer doesn't need to re-compile manually every time.

For more details about watch mode, see the Node.js API documentation or the CLI watch options.

MultiCompiler

This module, MultiCompiler, allows webpack to run multiple configurations in separate compiler. If the options parameter in the webpack's NodeJS api is an array of options, webpack applies separate compilers and calls the callback method at the end of each compiler execution.

var webpack = require('webpack');

var config1 = {
  entry: './index1.js',
  output: {filename: 'bundle1.js'}
}
var config2 = {
  entry: './index2.js',
  output: {filename:'bundle2.js'}
}

webpack([config1, config2], (err, stats) => {
  process.stdout.write(stats.toString() + "\n");
})

Event Hooks

This a reference guide to all the event hooks exposed by the Compiler.

Usage

Here's an example of an asynchronous emit event handler:

compiler.plugin("emit", function(compilation, callback) {
  // Do something async...
  setTimeout(function() {
    console.log("Done with async work...");
    callback();
  }, 1000);
});