[node] engines

1. 背景

npm 3.0.0之后package.json的engineStrict属性就不支持了，

This feature was removed in npm 3.0.0
Prior to npm 3.0.0, this feature was used to treat this package as if the user had set engine-strict. It is no longer used.

这样engines属性也就不支持了，

Unless the user has set the engine-strict config flag, this field is advisory only and will only produce warnings when your package is installed as a dependency.

因此就没办法指定模块的运行时node版本了。
不过我们还可以采用下面的办法实现。

2. 指定运行时node版本

2.1 先使用nvm安装最新的node LTS版本

$ nvm install --lts

$ node --version
10.14.1

$ npm --version
6.4.1

2.2 新建项目

$ npm init -f

2.3 安装指定的node版本作为devDependencies

$ npm i -S node@4.2.0

它会安装一个node@4.2.0模块到node_modules中，
并设置node_modules/.bin/node软链接到node_modules/node/bin/node。

$ tree -L 4 -a
.
├── node_modules
│   ├── .DS_Store
│   ├── .bin
│   │   └── node -> ../node/bin/node
│   ├── node
│   │   ├── .DS_Store
│   │   ├── README.md
│   │   ├── bin
│   │   │   └── node
│   │   ├── installArchSpecificPackage.js
│   │   ├── node_modules
│   │   │   ├── .bin
│   │   │   └── node-darwin-x64
│   │   └── package.json
│   └── node-bin-setup
│       ├── index.js
│       └── package.json
├── package-lock.json
└── package.json

2.4 新建源码文件index.js

let a = 1;

2.5 使用npx调用.bin中的命令

$ npx node index.js
~/Test/test-node/index.js:1
(function (exports, require, module, __filename, __dirname) { let a = 1;
                                                              ^^^

SyntaxError: Block-scoped declarations (let, const, function, class) not yet supported outside strict mode
    at exports.runInThisContext (vm.js:53:16)
    at Module._compile (module.js:414:25)
    at Object.Module._extensions..js (module.js:442:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:311:12)
    at Function.Module.runMain (module.js:467:10)
    at startup (node.js:134:18)
    at node.js:961:3

我们发现它报错了，
这是因为node@4.2.0不支持let，相关的issue可以参考这里。

3. npx

3.1 node_modules/.bin

npx是npm 5.2.0之后，默认全局安装的命令行工具，
它提供了一个简单的方式来调用node_modules/.bin下面的命令。

Executes <command> either from a local node_modules/.bin, or from a central cache, installing any packages needed in order for <command> to run.

By default, npx will check whether <command> exists in $PATH, or in the local project binaries, and execute that. If <command> is not found, it will be installed prior to execution.

有了npx，我们甚至还可以一条命令直接启动一个http-server，

$ npx http-server
npx: 25 安装成功，用时 8.057 秒
Starting up http-server, serving ./
Available on:
  http://127.0.0.1:8080
  http://10.15.230.10:8080
Hit CTRL-C to stop the server

npx会临时安装一个http-server模块，
并执行http-server模块中/.bin/文件夹中的http-server命令，
npx执行完后，临时的http-server模块会被删掉。

$ which http-server
http-server not found

3.2 npm scripts

如果不用npx，我们就得将命令加入到npm scripts中，例如，

{
  ...
  "scripts": {
    "dev": "node index.js"
  },
  ...
}

然后使用npm-run-script方式调用，

$ npm run dev

npm run会自动添加node_module/.bin 到当前命令所用的PATH变量中。

---

参考

engines
engineStrict
SyntaxError: Block-scoped declarations (let, const, function, class) not yet supported outside strict mode