本文参加了由公众号@若川视野 发起的每周源码共读活动，点击了解详情一起参与。

1、准备

克隆官方仓库
git clone https://github.com/pnpm/only-allow.git
cd only-allow
# npm i -g pnpm
pnpm i

1、看源码，先看read.md

Force a specific package manager to be used on a project  //强制在项目上使用特定的包管理器

Add a `preinstall` script to your project's `package.json`.

If you want to force [npm](https://docs.npmjs.com/cli/npm), add:

```json
{
  "scripts": {
    "preinstall": "npx only-allow npm"
  }
}

If you want to force [yarn](https://yarnpkg.com/), add:

```json
{
  "scripts": {
    "preinstall": "npx only-allow yarn"
  }
}

2、调试源码

// only-allow/package.json
{
  "bin": "bin.js",
}

提前在 only-allow/bin.js 文件打上断点 const usedPM = whichPMRuns()

按快捷键 ctrl + ` 快捷键打开终端。输入如下 yarn add release-it -D 命令，即可调试 only-allow/bin.js。

调试会报错

调试：新手向：前端程序员必学基本技能——调试JS代码

2、源码

#!/usr/bin/env node
const whichPMRuns = require('which-pm-runs')
const boxen = require('boxen')

const argv = process.argv.slice(2)
if (argv.length === 0) {
  console.log('Please specify the wanted package manager: only-allow <npm|cnpm|pnpm|yarn>')
  process.exit(1)
}
//用户传入的希望使用的包管理器
const wantedPM = argv[0]
//是不是包管理器
if (wantedPM !== 'npm' && wantedPM !== 'cnpm' && wantedPM !== 'pnpm' && wantedPM !== 'yarn') {
  console.log(`"${wantedPM}" is not a valid package manager. Available package managers are: npm, cnpm, pnpm, or yarn.`)
  process.exit(1)
}
const usedPM = whichPMRuns()
//不是期望的包管理器，报错
if (usedPM && usedPM.name !== wantedPM) {
  const boxenOpts = { borderColor: 'red', borderStyle: 'double', padding: 1 }
  switch (wantedPM) {
    case 'npm':
      console.log(boxen('Use "npm install" for installation in this project', boxenOpts))
      break
    case 'cnpm':
      console.log(boxen('Use "cnpm install" for installation in this project', boxenOpts))
      break
    case 'pnpm':
      console.log(boxen(`Use "pnpm install" for installation in this project.

If you don't have pnpm, install it via "npm i -g pnpm".
For more details, go to https://pnpm.js.org/`, boxenOpts))
      break
    case 'yarn':
      console.log(boxen(`Use "yarn" for installation in this project.

If you don't have Yarn, install it via "npm i -g yarn".
For more details, go to https://yarnpkg.com/`, boxenOpts))
      break
  }
  process.exit(1)
}

跟着断点，进入which-pm-runs

1、which-pm-runs 当前运行的是哪一个包管理器

返回包管理器和版本号

String,不推荐使用 substr。推荐使用 slice。