node-modules.md

CommonJS Modules

In this section, we mainly look at the CommonJS module system, which gets popular because of the Node.JS environment.

How to Export/Import Functionalities from a CommonJS Module

Exporting/Importing a Function

In module add.js

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

module.exports = add;

In module main.js

var adder = require('./add');

console.log(adder(1,100));

Exporting/Importing an Object

In module calculator.js

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

function subtract(x,y){
    return x-y;
}

module.exports = {
  add: add,
  sub: subtract
};

In module main.js

var utils = require('./utils');

console.log(utils.add(1,100));
console.log(utils.sub(100,1));

In module main.js, we could also import the add and sub function explicitly:

var {add, sub} = require('./utils');

console.log(add(1,100));
console.log(sub(100,1));

Note that this is not a new magic, we are just applying the object destructuring feature we have talked about in this case.

A Shortcut for module.exports

The above example in calculator.js can also be re-written as

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

function subtract(x,y){
    return x-y;
}

exports.add = add;
exports.sub = subtract;

The exports is an alias of module.exports (i.e. both point to the same object that is initialized before a module is loaded).

Note that you cannot re-assign the exports variable to another object. The following code won't work.

exports = {
  add: add,
  sub: subtract
};

You have to use module.exports instead of exports if you need to re-assign that variable to a different object.

The Node Module Registry

The NPM Registry is a place for people to publish their node modules. Anyone can do it. Even you can do it too!

Keep your node modules small

If you have a look at the node modules on the NPM registry, you can find that most of them are small, e.g.

• Most of them only do one thing, each of them is more like a function
• Each of them usually only have one small public interface, which is exposed via the module.exports object.

This follows the Unix philosophy of building tools - "Do One Thing and Do It Well".

Resources

• How the module system, CommonJS & require works