modules.md

JS Modules

module.exports I

We can get started with modules by defining a module in one file and making the module available for use in another file with Node.js module.exports syntax. Every JavaScript file run in Node has a local module object with an exports property used to define what should be exported from the file.

The pattern we use to export modules is thus:

1. Create an object to represent the module.
2. Add properties or methods to the module object.
3. Export the module with module.exports.

// 1-airplane.js
	let Airplane = {};
	Airplane.myAirplane = "StarJet";
	module.exports = Airplane;

let Airplane = {}; creates the object that represents the module Airplane and contains the var Airplane Airplane.myAirplane defines a property of the Airplane module we add data to the 'Airplane' object in this case 'StarJet' module.exports = Airplane; exports the Airplane object as a module, "module" is a variable that represents the module and "exports" exposes the module as an object.

module.exports II

We can also wrap any collection of data and functions in an object, and export the object using module.exports. In menu.js, we could write:

// menu.js
	module.exports = {
		specialty: "Roasted Beet Burger with Mint Sauce",
		getSpecialty: function() {
			return this.specialty;
		}
	};

Notice in the above code:

1. module.exports exposes the current module as an object.
2. specialty and getSpecialty are properties on the object.

Then in order.js we write:

	const Menu = require('./menu.js');
	console.log(Menu.getSpecialty());

Here we can still access the Menu module.

Import JS modules with the require() function

// 1-missionControl.js
	const Airplane =
	require('./1-airplane.js');

	function displayAirplane(){
	  console.log(Airplane.myAirplane);
	}

	displayAirplane(); // logs: StarJet in current file from 1-airplane.js

In Node.js the require function can be used to import code from another file into the current script.

	var moduleA = require("./module-a.js");
	// .js extension optional
	var moduleA = require("./module-a");

	// now the functionality of moduleA can be used
	console.log(moduleA.someFunctionality);

JS export default

As of ES6, the export default keywords allow for a single variable or function to be exported, then, in another script, it will be straightforward to import the default export. After using the export default it is possible to import a variable or function without using the require() function.

// module "moduleA.js"
	export default function cube(x) {
		return x * x * x;
	}
// In main.js
	import cube from './moduleA.js';
// now the cube function can be used straightforwardly.
	console.log(cube(3)); // 27	or 3 * 3 * 3

Using the import keyword in JS

As of the ES6, the import keyword can be used to import functions, objects or primitives previously exported into the current script.

There are many ways to use the import keyword, e.g. use * selector to import all the exports from a script: import * from 'module_name';.

A single function can be imported with curly brackets e.g. import {funcA} as name from 'module_name';

Or many functions by name: import {funcA, funcB} as name from 'module_name';

	// add.js
	export const add = (x,y) => {
		return x + y
	}

	// main.js
	import { add } from './add';
	console.log(add(2,3)); //5