Node.js Loading and using a module


A module can be "imported", or otherwise "required" by the require() function. For example, to load the http module that ships with Node.js, the following can be used:

const http = require('http');

Aside from modules that are shipped with the runtime, you can also require modules that you have installed from npm, such as express. If you had already installed express on your system via npm install express, you could simply write:

const express = require('express');

You can also include modules that you have written yourself as part of your application. In this case, to include a file named lib.js in the same directory as current file:

const mylib = require('./lib');

Note that you can omit the extension, and .js will be assumed. Once you load a module, the variable is populated with an object that contains the methods and properties published from the required file. A full example:

const http = require('http');

// The `http` module has the property `STATUS_CODES`
console.log(http.STATUS_CODES[404]); // outputs 'Not Found'

// Also contains `createServer()`
http.createServer(function(req, res) {
  res.writeHead(200, {'Content-Type': 'text/html'});
  res.write('<html><body>Module Test</body></html>');