Tutorial - Explaining the module export system of nodejs by example

Johan Coppieters - Lector Web Development Howest - Brugge

In this short note I try to explain the usage of the 'module.exports' object when using 'require' in a nodejs script. There is also a shorthand 'exports', which is an alias to 'module.export', don't use is, it is confusing.

In your module you can add something to the object 'module.exports'. The require function in a script using your module, will return this same object (with all the things in it, that you added, variables, functions, objects, ...)

in a module -> module.exports is a reference to an object

in a script -> require("module-filename") returns this object

Exporting functions

Defining a module

src/myModule.js

module.exports.turnOn = turnOn;
module.exports.turnOn = turnOff;
module.exports.isOn = isOn;

var switchState = false;

function turnOn() { switchState = true; } 
function turnOff() { switchState = false; }
function isOn() { switchState switch; }

Using this module

src/example.js

var aBulb = require("./myModule.js");

console.log("bulb is on " + aBulb.isOn());
aBulb.turnOn();
console.log("bulb is on " + aBulb.isOn());

The better object oriented way

If you want to return a single function from a module definition (for example a contructor function), you can directly set the exports variable

src/myModule1.js

module.exports = Lighting;

function Lighting() {
  this.switch = false;
}

Lighting.prototype.turnOn = function () {
  this.switch = true;
};
Lighting.prototype.turnOff = function () {
  this.switch = false;
};
Lighting.prototype.isOn = function () {
  return this.switch;
};

Using this module / object definition

src/example.js

var Bulb = require("./myModule1.js");

var aBulb = new Bulb();
console.log("bulb is on " + aBulb.isOn());
aBulb.turnOn();
console.log("bulb is on " + aBulb.isOn());

A complete library

If you have a number of modules you want to expose, put them all into a directory (example "myMods") and add a file index.js

src/myMods/index.js

module.export.module1 = require("./myModule1.js");
module.export.module2 = require("./myModule2.js");

In myModule 1 and 2.js just do as above.

Using these modules: in example.js which sits in a directory containing the myMods directory (if not, specify a different path, for example: "../libs/myMods")

src/example.js

var myMods = require("./myMods");

var aBulb = new myMods.Bulb();
console.log("bulb is on " + aBulb.isOn());
aBulb.turnOn();
console.log("bulb is on " + aBulb.isOn());