Modules

Modules are used to bring external functionality into your application.

var module1 = require('module1');

In the above statement, the require function loads a module and assigns it to a variable.

A module can export variables, functions and objects.

You can load as little functionality as you want from module as below:

var function1 = require('module1').function1;
console.log(function1());

Sources of Modules

Built-in Modules

var os = require('os');
console.log(os.hostname());
console.log(os.freemem());
console.log(os.totalmem());

Above ‘os’ module is a built-in module of Node.js

Custom Modules:

// File Name: math.js
const add = function(num1, num2) {
   console.log(num1 + num2);
}

module.exports = add;

• Writing a module is a great way to modularize your code. It will become easy to develop and maintain the application.
• A variable or a function can be exported using module.exports
• Variables and functions that are not exported will be inaccessible outside of this JavaScript file.

// File name: app.js
var math = require('./math');
math.add(2, 5);

• The module file can be loaded using require method having path of the module
• While loading the module, you can omit “.js”

Exporting multiple functions or variables:

• When you export using “module.exports” it export only 1 functioin/variable.
• When you want to export multiple functions/variables, you can write exports.function_name as below

// File Name: math.js
exports.add = function(num1, num2) {
   console.log(num1 + num2);
}

exports.subtract = function(num1, num2) {
   console.log(num1 - num2);
}

// File name: app.js
var math = require('./math');
math.add(2, 5);
math.subtract(2, 5);

Third-Party module

• Node Package Manager (npm) registry contains many third party node modules that you can download and use in your application
• Modules can be installed from npm registry using below command

npm install module_name

• Above command will download the module from npm registry into node_modules folder of your application.

my_app > node_module > module_name

• Once the module is installed, you can load it the same way you load a built-in module

const request = require('request');

request('https://techstackjournal.com/node-js/', function(error, response, body) {
    if(response.statusCode === 200) {
        console.log(body);
    }
});

• Some of the module provide command line utilities like gulp. They can be installed as below

npm install -g module_name

• This will install the module and the executables of this module are placed in PATH, so that they are available globally in your machine.

Various ways of requiring a module

const functionName = require('./module_name');

Looks for module_name.js in the current directory and loads it.

const functionName = require('../module_name');

Looks for module_name.js in the parent directory and loads it.

const functionName = require('module_name');

• Looks for module_name.js in the current directory and loads it.

Installing modules locally vs globally

	Installing module locally	Installing module globally
Command	npm install module_name	npm install module_name -g
Location	App > node_modules > module_name	npm_home > node_modules > module_name
require	You will be able to load the module using require in your application	Cannot load the module using require in your application
Command-line utility	Command-line utilities are accessible	Command-line utilities are accessible