Webapck packaging principle -- Analysis of startup process

One 、 Run the command

 （1）npm script

// development environment 
npm run dev
// Production environment 
npm run build

（2）webpack Direct execution

webpack entry.js bundle.js

Two 、 lookup Webpack Entrance file

（1） After running the above command on the command line ,npm Will let the command-line tool enter node_modules\.bin Find out whether the directory exists webpack.sh perhaps webpack.cmd file , If there is , Is executed , non-existent , Just throw the error .

（2） The actual entry file is ：node_modules\webpack\bin\webpack.js

3、 ... and 、webpack Entry file ：webpack.js analysis

#!/usr/bin/env node

// @ts-ignore
// Normal execution return （exitCode by 0,error by null）, Error reporting will be modified exitCode meanwhile throw error
process.exitCode = 0;

/**
 * @param {string} command process to run
 * @param {string[]} args commandline arguments
 * @returns {Promise<void>} promise
 */
// Run a command 
//command：npm
//arg: install xxx -d
// Provide a solution to quickly execute a command , There is no need to manually enter commands on the command line 
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?
 */
// Determine whether a package is installed 
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  Usable CLI:webpack-cli and webpack-command
/** @type {CliOption[]} */
const CLIs = [
  // It has rich features 
	{
		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."
	},
  // Relatively lighter 
	{
		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."
	}
];
// Whether two CLI Is... Installed 
const installedClis = CLIs.filter(cli => cli.installed);
// One is not installed 
if (installedClis.length === 0) {
	const path = require("path");
	const fs = require("fs");
	const readLine = require("readline");
// Error message 
	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);
  // Help with automatic installation 
	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(
			" "
		)}".`
	);
  // Ask if you want to install 
	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();
    // Whether to input 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;
		}
    // Start installation 
		const packageName = "webpack-cli";

		console.log(
			`Installing '${packageName}' (running '${packageManager} ${installOptions.join(
				" "
			)} ${packageName}')...`
		);
    // Execute the installation command 
		runCommand(packageManager, installOptions.concat(packageName))
			.then(() => {
				require(packageName); //eslint-disable-line
			})
			.catch(error => {
				console.error(error);
				process.exitCode = 1;
			});
	});
  // Installed a , Use it directly 
} else if (installedClis.length === 1) {
	const path = require("path");
	const pkgPath = require.resolve(`${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 {
  // Two installed , Prompt to delete one before running 
	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;
}

  Four 、 Results after startup

webpack Finally find webpack-cli (webpack-command) This npm package , And perform CLI.