webapck打包原理--启动过程分析

一、运行命令

 （1）npm script

//开发环境
npm run dev
//生产环境
npm run build

（2）webpack直接执行

webpack entry.js bundle.js

二、查找Webpack入口文件

（1）在命令行运行以上命令后，npm会让命令行工具进入node_modules\.bin 目目录查找是否存在 webpack.sh 或者 webpack.cmd 文件，如果存在，就执行，不存在，就抛出错误。

（2）实际的入口文件是：node_modules\webpack\bin\webpack.js

三、webpack 的入口文件：webpack.js 分析

#!/usr/bin/env node

// @ts-ignore
//正常执行返回（exitCode为0，error为null），报错会修改exitCode同时throw error
process.exitCode = 0;

/**
 * @param {string} command process to run
 * @param {string[]} args commandline arguments
 * @returns {Promise<void>} promise
 */
//运行某个命令
//command：npm
//arg: install xxx -d
//提供一个方案快速的去执行一个命令，而不需要再命令行里手动地去输入命令
const runCommand = (command, args) => {
	const cp = require("child_process");
	return new Promise((resolve, reject) => {
		const executedCommand = cp.spawn(command, args, {
			stdio: "inherit",
			shell: true
		});

		executedCommand.on("error", error => {
			reject(error);
		});

		executedCommand.on("exit", code => {
			if (code === 0) {
				resolve();
			} else {
				reject();
			}
		});
	});
};

/**
 * @param {string} packageName name of the package
 * @returns {boolean} is the package installed?
 */
//判断某个包是否安装
const isInstalled = packageName => {
	try {
		require.resolve(packageName);

		return true;
	} catch (err) {
		return false;
	}
};

/**
 * @typedef {Object} CliOption
 * @property {string} name display name
 * @property {string} package npm package name
 * @property {string} binName name of the executable file
 * @property {string} alias shortcut for choice
 * @property {boolean} installed currently installed?
 * @property {boolean} recommended is recommended
 * @property {string} url homepage
 * @property {string} description description
 */
//webpack 可用的CLI:webpack-cli和webpack-command
/** @type {CliOption[]} */
const CLIs = [
  //具有比较丰富的特性
	{
		name: "webpack-cli",
		package: "webpack-cli",
		binName: "webpack-cli",
		alias: "cli",
		installed: isInstalled("webpack-cli"),
		recommended: true,
		url: "https://github.com/webpack/webpack-cli",
		description: "The original webpack full-featured CLI."
	},
  //相对来说更加轻量
	{
		name: "webpack-command",
		package: "webpack-command",
		binName: "webpack-command",
		alias: "command",
		installed: isInstalled("webpack-command"),
		recommended: false,
		url: "https://github.com/webpack-contrib/webpack-command",
		description: "A lightweight, opinionated webpack CLI."
	}
];
//判断两个CLI是否安装了
const installedClis = CLIs.filter(cli => cli.installed);
//一个没有安装
if (installedClis.length === 0) {
	const path = require("path");
	const fs = require("fs");
	const readLine = require("readline");
//错误提示
	let notify =
		"One CLI for webpack must be installed. These are recommended choices, delivered as separate packages:";

	for (const item of CLIs) {
		if (item.recommended) {
			notify += `\n - ${item.name} (${item.url})\n   ${item.description}`;
		}
	}

	console.error(notify);
  //提供帮助自动安装
	const isYarn = fs.existsSync(path.resolve(process.cwd(), "yarn.lock"));

	const packageManager = isYarn ? "yarn" : "npm";
	const installOptions = [isYarn ? "add" : "install", "-D"];

	console.error(
		`We will use "${packageManager}" to install the CLI via "${packageManager} ${installOptions.join(
			" "
		)}".`
	);
  //询问是否安装
	const question = `Do you want to install 'webpack-cli' (yes/no): `;

	const questionInterface = readLine.createInterface({
		input: process.stdin,
		output: process.stderr
	});
	questionInterface.question(question, answer => {
		questionInterface.close();
    //是否输入yes
		const normalizedAnswer = answer.toLowerCase().startsWith("y");

		if (!normalizedAnswer) {
			console.error(
				"You need to install 'webpack-cli' to use webpack via CLI.\n" +
					"You can also install the CLI manually."
			);
			process.exitCode = 1;

			return;
		}
    //开始安装
		const packageName = "webpack-cli";

		console.log(
			`Installing '${packageName}' (running '${packageManager} ${installOptions.join(
				" "
			)} ${packageName}')...`
		);
    //执行安装命令
		runCommand(packageManager, installOptions.concat(packageName))
			.then(() => {
				require(packageName); //eslint-disable-line
			})
			.catch(error => {
				console.error(error);
				process.exitCode = 1;
			});
	});
  //安装了一个，直接使用
} else if (installedClis.length === 1) {
	const path = require("path");
	const pkgPath = require.resolve(`${installedClis[0].package}/package.json`);
	// eslint-disable-next-line node/no-missing-require
	const pkg = require(pkgPath);
	// eslint-disable-next-line node/no-missing-require
	require(path.resolve(
		path.dirname(pkgPath),
		pkg.bin[installedClis[0].binName]
	));
} else {
  //安装了两个，提示要删掉一个才可以运行
	console.warn(
		`You have installed ${installedClis
			.map(item => item.name)
			.join(
				" and "
			)} together. To work with the "webpack" command you need only one CLI package, please remove one of them or use them directly via their binary.`
	);

	// @ts-ignore
	process.exitCode = 1;
}

 四、启动后的结果

webpack 最终找到 webpack-cli (webpack-command) 这个 npm 包，并且执行 CLI。