以react初始化项目为例：
git clone [email protected]:geektcp/react-init.git
package.json的内容如下：
{
"name": "react-init",
"version": "0.1.0",
"private": true,
"dependencies": {
"@testing-library/jest-dom": "^5.16.5",
"@testing-library/react": "^13.4.0",
"@testing-library/user-event": "^13.5.0",
"react": "^18.2.0",
"react-dom": "^18.2.0",
"react-scripts": "5.0.1",
"web-vitals": "^2.1.4"
},
"scripts": {
"start": "react-scripts start",
"build": "react-scripts build",
"test": "react-scripts test",
"eject": "react-scripts eject"
},
"eslintConfig": {
"extends": [
"react-app",
"react-app/jest"
]
},
"browserslist": {
"production": [
">0.2%",
"not dead",
"not op_mini all"
],
"development": [
"last 1 chrome version",
"last 1 firefox version",
"last 1 safari version"
]
}
}
npm start其实是一个系统级别的命令，开发人员需要安装npm 并配置好环境变量。
这个命令可以理解为一个shell脚本，进入当前目录的package.json中找到start对应的脚本并执行。把package.json中start对应脚本换成任意脚本或者命令都可以直接执行。
这里是对应的脚本是：
react-scripts start
react-scripts其实是项目目录下一个文件，具体路径是：./node_modules/react-scripts，内容如下：
#!/usr/bin/env node
/**
* Copyright (c) 2015-present, Facebook, Inc.
*
* This source code is licensed under the MIT license found in the
* LICENSE file in the root directory of this source tree.
*/
'use strict';
// Makes the script crash on unhandled rejections instead of silently
// ignoring them. In the future, promise rejections that are not handled will
// terminate the Node.js process with a non-zero exit code.
process.on('unhandledRejection', err => {
throw err;
});
const spawn = require('react-dev-utils/crossSpawn');
const args = process.argv.slice(2);
const scriptIndex = args.findIndex(
x => x === 'build' || x === 'eject' || x === 'start' || x === 'test'
);
const script = scriptIndex === -1 ? args[0] : args[scriptIndex];
const nodeArgs = scriptIndex > 0 ? args.slice(0, scriptIndex) : [];
if (['build', 'eject', 'start', 'test'].includes(script)) {
const result = spawn.sync(
process.execPath,
nodeArgs
.concat(require.resolve('../scripts/' + script))
.concat(args.slice(scriptIndex + 1)),
{ stdio: 'inherit' }
);
if (result.signal) {
if (result.signal === 'SIGKILL') {
console.log(
'The build failed because the process exited too early. ' +
'This probably means the system ran out of memory or someone called ' +
'`kill -9` on the process.'
);
} else if (result.signal === 'SIGTERM') {
console.log(
'The build failed because the process exited too early. ' +
'Someone might have called `kill` or `killall`, or the system could ' +
'be shutting down.'
);
}
process.exit(1);
}
process.exit(result.status);
} else {
console.log('Unknown script "' + script + '".');
console.log('Perhaps you need to update react-scripts?');
console.log(
'See: https://facebook.github.io/create-react-app/docs/updating-to-new-releases'
);
}
如上，见标注红色字体的部分，关键是spawn.sync这句,，其实就是执行一个js文件，
其路径为：./node_modules/react-scripts/scripts/start.js，内容如下：
// @remove-on-eject-begin
/**
* Copyright (c) 2015-present, Facebook, Inc.
*
* This source code is licensed under the MIT license found in the
* LICENSE file in the root directory of this source tree.
*/
// @remove-on-eject-end
'use strict';
// Do this as the first thing so that any code reading it knows the right env.
process.env.BABEL_ENV = 'development';
process.env.NODE_ENV = 'development';
// Makes the script crash on unhandled rejections instead of silently
// ignoring them. In the future, promise rejections that are not handled will
// terminate the Node.js process with a non-zero exit code.
process.on('unhandledRejection', err => {
throw err;
});
// Ensure environment variables are read.
require('../config/env');
const fs = require('fs');
const chalk = require('react-dev-utils/chalk');
const webpack = require('webpack');
const WebpackDevServer = require('webpack-dev-server');
const clearConsole = require('react-dev-utils/clearConsole');
const checkRequiredFiles = require('react-dev-utils/checkRequiredFiles');
const {
choosePort,
createCompiler,
prepareProxy,
prepareUrls,
} = require('react-dev-utils/WebpackDevServerUtils');
const openBrowser = require('react-dev-utils/openBrowser');
const semver = require('semver');
const paths = require('../config/paths');
const configFactory = require('../config/webpack.config');
const createDevServerConfig = require('../config/webpackDevServer.config');
const getClientEnvironment = require('../config/env');
const react = require(require.resolve('react', { paths: [paths.appPath] }));
const env = getClientEnvironment(paths.publicUrlOrPath.slice(0, -1));
const useYarn = fs.existsSync(paths.yarnLockFile);
const isInteractive = process.stdout.isTTY;
// Warn and crash if required files are missing
if (!checkRequiredFiles([paths.appHtml, paths.appIndexJs])) {
process.exit(1);
}
// Tools like Cloud9 rely on this.
const DEFAULT_PORT = parseInt(process.env.PORT, 10) || 3000;
const HOST = process.env.HOST || '0.0.0.0';
if (process.env.HOST) {
console.log(
chalk.cyan(
`Attempting to bind to HOST environment variable: ${chalk.yellow(
chalk.bold(process.env.HOST)
)}`
)
);
console.log(
`If this was unintentional, check that you haven't mistakenly set it in your shell.`
);
console.log(
`Learn more here: ${chalk.yellow('https://cra.link/advanced-config')}`
);
console.log();
}
// We require that you explicitly set browsers and do not fall back to
// browserslist defaults.
const { checkBrowsers } = require('react-dev-utils/browsersHelper');
checkBrowsers(paths.appPath, isInteractive)
.then(() => {
// We attempt to use the default port but if it is busy, we offer the user to
// run on a different port. `choosePort()` Promise resolves to the next free port.
return choosePort(HOST, DEFAULT_PORT);
})
.then(port => {
if (port == null) {
// We have not found a port.
return;
}
const config = configFactory('development');
const protocol = process.env.HTTPS === 'true' ? 'https' : 'http';
const appName = require(paths.appPackageJson).name;
const useTypeScript = fs.existsSync(paths.appTsConfig);
const urls = prepareUrls(
protocol,
HOST,
port,
paths.publicUrlOrPath.slice(0, -1)
);
// Create a webpack compiler that is configured with custom messages.
const compiler = createCompiler({
appName,
config,
urls,
useYarn,
useTypeScript,
webpack,
});
// Load proxy config
const proxySetting = require(paths.appPackageJson).proxy;
const proxyConfig = prepareProxy(
proxySetting,
paths.appPublic,
paths.publicUrlOrPath
);
// Serve webpack assets generated by the compiler over a web server.
const serverConfig = {
...createDevServerConfig(proxyConfig, urls.lanUrlForConfig),
host: HOST,
port,
};
const devServer = new WebpackDevServer(serverConfig, compiler);
// Launch WebpackDevServer.
devServer.startCallback(() => {
if (isInteractive) {
clearConsole();
}
if (env.raw.FAST_REFRESH && semver.lt(react.version, '16.10.0')) {
console.log(
chalk.yellow(
`Fast Refresh requires React 16.10 or higher. You are using React ${react.version}.`
)
);
}
console.log(chalk.cyan('Starting the development server...\n'));
openBrowser(urls.localUrlForBrowser);
});
['SIGINT', 'SIGTERM'].forEach(function (sig) {
process.on(sig, function () {
devServer.close();
process.exit();
});
});
if (process.env.CI !== 'true') {
// Gracefully exit when stdin ends
process.stdin.on('end', function () {
devServer.close();
process.exit();
});
}
})
.catch(err => {
if (err && err.message) {
console.log(err.message);
}
process.exit(1);
});
这部分最关键的是这句：
const devServer = new WebpackDevServer(serverConfig, compiler);
// Launch WebpackDevServer.
devServer.startCallback(() => {
if (isInteractive) {
clearConsole();
}
就是new一个WebpackDevServer服务器，类似启动一个nginx服务器一样，启动一个web服务器。
总结一下npm start的引导过程：
npm start
-> ./node_modules/react-scripts/bin/react-scripts start
-> ./node_modules/react-scripts/scripts/start.js
-> (new WebpackDevServer (serverConfig,compiler)).startCallback
标签：npm,const,process,require,react,start,scripts,前端开发 From： https://www.cnblogs.com/geektcp/p/16953694.html