Node Modules

Context

We are now executing our code in a different context, one that no longer relies on <script> and index.html.

If we want one set of javascript inside file A to interact with other javascript in file B, how do we accomplish that?

The answer is modules.

Objectives

• Explain the use of modules
• Code a module

Node Modules

How do we include other javascript into our node.js programs? We are used to script and link.

If we wanted to have a second or third javascript file accessible to our original file, or CSS that adds or modifies our current CSS, we could just add it:

<script src="script2.js"></script>

<link rel="stylesheet" href="style2.css">

But our javascript in no longer being executed in the context of a web page.

Luckily node provides a way to use external libraries.

// the jquery library is actually all contained in the variable $
const myOtherCode = require('script2.js');

Let's see how this works:

Make your own modules

In essence, if a file puts something inside of module.exports, it can be made available for use in any other file using require().

For example, let's make two files: touch my-module.js main.js

my-module.js:

// declare a variable in the file
const number = 7;

// add a key and a string to the exports object
module.exports.name = "Kenaniah";


// add an array to the exports object
module.exports.arr = [1, 2, 3];

// add a function to the exports object
module.exports.getNumber = function(){

    // try to access the variable defined above.
    console.log("Get number called. Returning: ", number);
    return number;
}

console.log("End of my-module.js file")

Use the module you created

main.js:

// here we're grabbing everything that's "exported" in our other file, and storing it a variable called 'my'
const my = require('./my-module')

// Variables and such that were not exported aren't in scope
console.log("number is " + typeof number) // undefined

// Anything exported can be accessed on the object
console.log("Name is: ", my.name)

// JavaScript is still JavaScript
console.log("The array contains " + my.arr.length + " elements")

// Let's see the module we imported
console.log(my)

Then try running:

node my-module.js
node main.js

Pairing Exercise

Run the above code.

Further

Write a set of calculator functions in a file called calculator.js. (add, subtract, multiply, divide)

Export all the functions.

In a new file index.js write a command line program that adds 2 + 2 using the functons defined in calculator.js.

Further

Make the command line program take arguments from the comand line.

Ex:

node index.js add 2 9

Further

Write a 3rd file, circle.js. This one requires calculator.js to calculate dimensions of a circle. (area, circumference, etc.)

Add to index.js to call the functions you wrote in circle.js.

node index.js circle area 7

node index.js circumference 7