webpack/lib/optimize/AggressiveMergingPlugin.js

Packs ECMAScript/CommonJs/AMD modules for the browser. Allows you to split your codebase into multiple bundles, which can be loaded on demand. Supports loaders to preprocess files, i.e. json, jsx, es7, css, less, ... and your custom stuff.

/*
	MIT License http://www.opensource.org/licenses/mit-license.php
	Author Tobias Koppers @sokra
*/

"use strict";

const { STAGE_ADVANCED } = require("../OptimizationStages");

/** @typedef {import("../Chunk")} Chunk */
/** @typedef {import("../Compiler")} Compiler */

/**
 * Defines the aggressive merging plugin options type used by this module.
 * @typedef {object} AggressiveMergingPluginOptions
 * @property {number=} minSizeReduce minimal size reduction to trigger merging
 */

const PLUGIN_NAME = "AggressiveMergingPlugin";

class AggressiveMergingPlugin {
	/**
	 * Creates an instance of AggressiveMergingPlugin.
	 * @param {AggressiveMergingPluginOptions=} options options object
	 */
	constructor(options) {
		if (
			(options !== undefined && typeof options !== "object") ||
			Array.isArray(options)
		) {
			throw new Error(
				"Argument should be an options object. To use defaults, pass in nothing.\nFor more info on options, see https://webpack.js.org/plugins/"
			);
		}
		/** @type {AggressiveMergingPluginOptions} */
		this.options = options || {};
	}

	/**
	 * Applies the plugin by registering its hooks on the compiler.
	 * @param {Compiler} compiler the compiler instance
	 * @returns {void}
	 */
	apply(compiler) {
		const options = this.options;
		const minSizeReduce = options.minSizeReduce || 1.5;

		compiler.hooks.thisCompilation.tap(PLUGIN_NAME, (compilation) => {
			compilation.hooks.optimizeChunks.tap(
				{
					name: PLUGIN_NAME,
					stage: STAGE_ADVANCED
				},
				(chunks) => {
					const chunkGraph = compilation.chunkGraph;
					// Only the single best pair is merged per pass (the hook re-runs
					// after each merge), so track the max directly instead of building
					// and sorting the full O(chunks²) list of pairs every pass. A strict
					// `>` keeps the first-encountered best, matching the previous stable
					// descending sort's `[0]`.
					/** @type {Chunk | undefined} */
					let bestA;
					/** @type {Chunk | undefined} */
					let bestB;
					let bestImprovement = -Infinity;
					for (const a of chunks) {
						if (a.canBeInitial()) continue;
						for (const b of chunks) {
							if (b.canBeInitial()) continue;
							if (b === a) break;
							if (!chunkGraph.canChunksBeIntegrated(a, b)) {
								continue;
							}
							const aSize = chunkGraph.getChunkSize(b, {
								chunkOverhead: 0
							});
							const bSize = chunkGraph.getChunkSize(a, {
								chunkOverhead: 0
							});
							const abSize = chunkGraph.getIntegratedChunksSize(b, a, {
								chunkOverhead: 0
							});
							const improvement = (aSize + bSize) / abSize;
							if (improvement > bestImprovement) {
								bestImprovement = improvement;
								bestA = a;
								bestB = b;
							}
						}
					}

					if (bestA === undefined) return;
					if (bestImprovement < minSizeReduce) return;

					chunkGraph.integrateChunks(/** @type {Chunk} */ (bestB), bestA);
					compilation.chunks.delete(bestA);
					return true;
				}
			);
		});
	}
}

module.exports = AggressiveMergingPlugin;