CommonJS 模块

CommonJS 模块是为 Node.js 打包 JavaScript 代码的原始方式。 Node.js 还支持浏览器和其他 JavaScript 运行时使用的 ECMAScript 模块标准。

在 Node.js 中，每个文件都被视为一个单独的模块。 例如，假设一个名为 foo.js 的文件：

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

在第一行，foo.js 加载了与 foo.js 位于同一目录中的模块 circle.js。

以下是 circle.js 的内容：

const { PI } = Math;

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

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

模块 circle.js 已导出函数 area() 和 circumference()。 通过在特殊的 exports 对象上指定额外的属性，将函数和对象添加到模块的根部。

模块的本地变量将是私有的，因为模块被 Node.js 封装在函数中（参见模块封装器）。 在此示例中，变量 PI 是 circle.js 私有的。

可以为 module.exports 属性分配新的值（例如函数或对象）。

下面，bar.js 使用了导出 Square 类的 square 模块：

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

square 模块在 square.js 中定义：

// 赋值给 exports 不会修改模块，必须使用 module.exports
module.exports = class Square {
  constructor(width) {
    this.width = width;
  }

  area() {
    return this.width ** 2;
  }
};

CommonJS 模块系统在 module 核心模块中实现。

CommonJS modules are the original way to package JavaScript code for Node.js. Node.js also supports the ECMAScript modules standard used by browsers and other JavaScript runtimes.

In Node.js, each file is treated as a separate module. For example, consider a file named foo.js:

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

On the first line, foo.js loads the module circle.js that is in the same directory as foo.js.

Here are the contents of circle.js:

const { PI } = Math;

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

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

The module circle.js has exported the functions area() and circumference(). Functions and objects are added to the root of a module by specifying additional properties on the special exports object.

Variables local to the module will be private, because the module is wrapped in a function by Node.js (see module wrapper). In this example, the variable PI is private to circle.js.

The module.exports property can be assigned a new value (such as a function or object).

Below, bar.js makes use of the square module, which exports a Square class:

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

The square module is defined in square.js:

// Assigning to exports will not modify module, must use module.exports
module.exports = class Square {
  constructor(width) {
    this.width = width;
  }

  area() {
    return this.width ** 2;
  }
};

The CommonJS module system is implemented in the module core module.