const circle = require('./circle.js');
console.log(`The area of a circle of radius 4 is ${circle.area(4)}`);

const PI = Math.PI;

exports.area = (r) => PI * r * r;

exports.circumference = (r) => 2* PI * r;

const square = require('./square.js');
var mySquare = square(2);
console.log(`The area of my square is ${mySquare.area()}`);

module.exports = (width) => {
  return {
    area: () => width * width;
  };
}

// 加载X模块
require(X) from module at path Y
1. If X is a core module.
  a. return the core module
  b. STOP
2. If X begins with './' or '/' or '../'
  a. LOAD_AS_FILE(Y + X)
  b. LOAD_AS_DIRECTORY(Y + X)
3. LOAD_NODE_MODULES(X, dirname(Y))
4. THROW "not found"

// 加载X文件
// 加载过程：X -> X.js -> X.json -> X.node
LOAD_AS_FILE(X)
1. If [X] is a file, load [X] as JavaScript text. STOP
2. If [X.js] is a file, load [X.js] as JavaScript text. STOP
3. If [X.json] is a file, load [X.json] as JavaScript text. STOP
4. If [X.node] is a file, load [X.node] as JavaScript text. STOP

// 加载入口文件
// 加载过程：X -> X/index.js -> X/index.json -> X/index.node
LOAD_INDEX(X)
1. If [X/index.js] is a file, load [X/index.js] as JavaScript text. STOP
2. If [X/index.json] is a file, load [X/index.json] as JavaScript text. STOP
3. If [X/index.node] if a file, load [X/index.node] as JavaScript text. STOP

// 加载文件夹
LOAD_AS_DIRECTORY(X)
1. If [X/package.json] is a file.
  a. Parse [X/package.json], and look for "main" field
  b. let M = X + (json main field)
  c. LOAD_AS_FILE(M)
  d. LOAD_INDEX(M)
2. LOAD_INDEX(X)
 
// 加载node模块
LOAD_NODE_MODULES(X, START)
1. let DIRS = NODE_MODULES_PATHS(START)
2. for each DIR in DIRS;
  a. LOAD_AS_FILE(DIR/X)
  b. LOAD_AS_DIRECTORY(DIR/X)

// 列出所有可能的node_modules路径
NODE_MODULES_PATHS(START)
1. let PARTS = path split(START);
2. let I = count of PARTS - 1
3. let DIRS = []
4. while I > 0
  a. If PARTS[I] = "node_modules" CONTINUE
  b. DIR = path join(PARTS[0 ... I] + "node_modules")
  c. DIRS = DIRS + DIR
  d. let I = I -1
5. return DIRS

console.log('a starting');
exports.done = false;
const b = require('./b.js');
console.log('in a, b.done = %j', b.done);
exports.done = true;
console.log('a done');

console.log('b starting');
exports.done = false;
const a = require('./a.js');
console.log('in b, a.done = %j', a.done);
exports.done = true;
console.log('b done');

console.log('main starting');
const a = require('./a.js');
const b = require('./b.js');
console.log('in main, a.done = %j, b.done = %j', a.done, b.done);

$ node app.js
main starting
a starting
b starting
in b, a.done = false
b done
in a, b.done = true
in main, a.done =true, b.done = true

(function (exports, require, module, __filename, __dirname) {
  // 你的模块代码
});

Object module {
  id: String, // 模块标识，为该模块文件在系统中的绝对路径
  exports: Object, // 该模块的导出对象
  parent: Object | undefined, // 引用该模块的父模块
  filename: String | null, // 最终解析的文件名称， 与__filename相同。
  loaded: Boolean, // 该模块是否已经加载
  children: Array, // 改模块的引用列表
  paths: Array // 模块加载路径
}

Function require {
  [Function], // 函数体
  resolve: Function, // 根据模块标识解析模块，返回绝对路径
  main: undefined | Object, // 应用的主(main)模块
  extensions: {'.js':Function, '.json':Function, '.node':Function},
  cache: Object // 模块缓存，以模块的绝对路径为key
}