source: frontend/node_modules/webpack/lib/AutomaticPrefetchPlugin.js

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

"use strict";

const asyncLib = require("neo-async");
const NormalModule = require("./NormalModule");
const PrefetchDependency = require("./dependencies/PrefetchDependency");

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

const PLUGIN_NAME = "AutomaticPrefetchPlugin";

/**
 * Records modules from one compilation and adds them back as prefetch
 * dependencies in the next compilation.
 */
class AutomaticPrefetchPlugin {
        /**
         * Registers hooks that remember previously built normal modules and enqueue
         * them as `PrefetchDependency` requests during the next make phase.
         * @param {Compiler} compiler the compiler instance
         * @returns {void}
         */
        apply(compiler) {
                compiler.hooks.compilation.tap(
                        PLUGIN_NAME,
                        (compilation, { normalModuleFactory }) => {
                                compilation.dependencyFactories.set(
                                        PrefetchDependency,
                                        normalModuleFactory
                                );
                        }
                );
                /** @type {{ context: string | null, request: string }[] | null} */
                let lastModules = null;
                compiler.hooks.afterCompile.tap(PLUGIN_NAME, (compilation) => {
                        lastModules = [];

                        for (const m of compilation.modules) {
                                if (m instanceof NormalModule) {
                                        lastModules.push({
                                                context: m.context,
                                                request: m.request
                                        });
                                }
                        }
                });
                compiler.hooks.make.tapAsync(PLUGIN_NAME, (compilation, callback) => {
                        if (!lastModules) return callback();
                        asyncLib.each(
                                lastModules,
                                (m, callback) => {
                                        compilation.addModuleChain(
                                                m.context || compiler.context,
                                                new PrefetchDependency(`!!${m.request}`),
                                                callback
                                        );
                                },
                                (err) => {
                                        lastModules = null;
                                        callback(err);
                                }
                        );
                });
        }
}

module.exports = AutomaticPrefetchPlugin;