模块是什么？

在nodejs中每一个文件都是一个模块，默认含有2个属性，module和require

示例

function add (x,y){
  return x+y
}

console.log(module)
console.log(require)

module

提示

有exports属性

Module {
  id: '.',
  path: '/Users/fangzhiqing/Desktop/fangdown/github/app-node/src',
  exports: {},
  parent: null,
  filename: '/Users/fangzhiqing/Desktop/fangdown/github/app-node/src/test.js',
  loaded: false,
  children: [],
  paths: [
    '/Users/fangzhiqing/Desktop/fangdown/github/app-node/src/node_modules',
    '/Users/fangzhiqing/Desktop/fangdown/github/app-node/node_modules',
    '/Users/fangzhiqing/Desktop/fangdown/github/node_modules',
    '/Users/fangzhiqing/Desktop/fangdown/node_modules',
    '/Users/fangzhiqing/Desktop/node_modules',
    '/Users/fangzhiqing/node_modules',
    '/Users/node_modules',
    '/node_modules'
  ]
}

require

有cache属性，缓存模块

[Function: require] {
  resolve: [Function: resolve] { paths: [Function: paths] },
  main: Module {
    id: '.',
    path: '/Users/fangzhiqing/Desktop/fangdown/github/app-node/src',
    exports: {},
    parent: null,
    filename: '/Users/fangzhiqing/Desktop/fangdown/github/app-node/src/test.js',
    loaded: false,
    children: [],
    paths: [
      '/Users/fangzhiqing/Desktop/fangdown/github/app-node/src/node_modules',
      '/Users/fangzhiqing/Desktop/fangdown/github/app-node/node_modules',
      '/Users/fangzhiqing/Desktop/fangdown/github/node_modules',
      '/Users/fangzhiqing/Desktop/fangdown/node_modules',
      '/Users/fangzhiqing/Desktop/node_modules',
      '/Users/fangzhiqing/node_modules',
      '/Users/node_modules',
      '/node_modules'
    ]
  },
  extensions: [Object: null prototype] {
    '.js': [Function],
    '.json': [Function],
    '.node': [Function]
  },
  cache: [Object: null prototype] {
    '/Users/fangzhiqing/Desktop/fangdown/github/app-node/src/test.js': Module {
      id: '.',
      path: '/Users/fangzhiqing/Desktop/fangdown/github/app-node/src',
      exports: {},
      parent: null,
      filename: '/Users/fangzhiqing/Desktop/fangdown/github/app-node/src/test.js',
      loaded: false,
      children: [],
      paths: [Array]
    }
  }
}

模块载入和缓存机制

1. 加载内置模块
2. 加载文件模块
3. 加载文件目录模块
4. 加载node_modules里的模块
5. 自动缓存已载入的模块（优先）

文件定位

没有扩展名，会按.js ---> .json ---> .node的顺序补足扩展名，依次尝试