initCompiler

const webpack = /** @type {WebpackFunctionSingle & WebpackFunctionMulti} */ (
	/**
	 * @param {WebpackOptions | (ReadonlyArray<WebpackOptions> & MultiCompilerOptions)} options options
	 * @param {Callback<Stats> & Callback<MultiStats>=} callback callback
	 * @returns {Compiler | MultiCompiler}
	 */
	(options, callback) => {
		const create = () => {
			if (!asArray(options).every(webpackOptionsSchemaCheck)) {
				getValidateSchema()(webpackOptionsSchema, options);
				util.deprecate(
					() => {},
					"webpack bug: Pre-compiled schema reports error while real schema is happy. This has performance drawbacks.",
					"DEP_WEBPACK_PRE_COMPILED_SCHEMA_INVALID"
				)();
			}
			/** @type {MultiCompiler|Compiler} */
			let compiler;
			let watch = false;
			/** @type {WatchOptions|WatchOptions[]} */
			let watchOptions;
			if (Array.isArray(options)) {
				/** @type {MultiCompiler} */
				compiler = createMultiCompiler(
					options,
					/** @type {MultiCompilerOptions} */ (options)
				);
				watch = options.some(options => options.watch);
				watchOptions = options.map(options => options.watchOptions || {});
			} else {
				const webpackOptions = /** @type {WebpackOptions} */ (options);
				/** @type {Compiler} */
				compiler = createCompiler(webpackOptions); // 创建 compiler
				watch = webpackOptions.watch;
				watchOptions = webpackOptions.watchOptions || {};
			}
			return { compiler, watch, watchOptions };
		};
		if (callback) {
			try {
				const { compiler, watch, watchOptions } = create();
				if (watch) {
					compiler.watch(watchOptions, callback);
				} else {
					compiler.run((err, stats) => {
						compiler.close(err2 => {
							callback(err || err2, stats);
						});
					});
				}
				return compiler;
			} catch (err) {
				process.nextTick(() => callback(err));
				return null;
			}
		} else {
			const { compiler, watch } = create();
			if (watch) {
				util.deprecate(
					() => {},
					"A 'callback' argument needs to be provided to the 'webpack(options, callback)' function when the 'watch' option is set. There is no way to handle the 'watch' option without a callback.",
					"DEP_WEBPACK_WATCH_WITHOUT_CALLBACK"
				)();
			}
			return compiler;
		}
	}
);

/**
 * @param {WebpackOptions} rawOptions options object
 * @returns {Compiler} a compiler
 */
const createCompiler = rawOptions => {
	const options = getNormalizedWebpackOptions(rawOptions);
	applyWebpackOptionsBaseDefaults(options);
	const compiler = new Compiler(options.context, options); // 初始化 compiler
	new NodeEnvironmentPlugin({
		infrastructureLogging: options.infrastructureLogging
	}).apply(compiler); // 为 compiler 添加文件处理方法
    // 注册插件
	if (Array.isArray(options.plugins)) {
		for (const plugin of options.plugins) {
			if (typeof plugin === "function") {
				plugin.call(compiler, compiler);
			} else {
				plugin.apply(compiler);
			}
		}
	}
	applyWebpackOptionsDefaults(options); // options 默认值处理
	compiler.hooks.environment.call(); // webpack 环境准备好触发
	compiler.hooks.afterEnvironment.call(); // environment 安装完成之后，执行插件。
	new WebpackOptionsApply().process(options, compiler);
	compiler.hooks.initialize.call();
	return compiler;
};