Skip to main content Accessibility Feedback

The vanilla JS Class pattern

The other day, we looked at the JavaScript Constructor Pattern, and how you can use it to create JS libraries.

Today, I wanted to look at the JavaScript Class Pattern: what it is, how it’s different, and how to use it. This article is excerpt from my completely updated Writing JS Libraries course and ebook.

Let’s dig in!

(If you didn’t read the constructor pattern article, you should do that first. Otherwise, a lot of today’s article won’t make much sense.)

What are JavaScript classes?

JavaScript classes provide an alternative way to implement constructor pattern libraries.

They use prototypes under-the-hood, but include some features and syntax that a traditional constructor pattern does not have.

To create a JavaScript class, you use the class keyword followed by the name of your class. Just like with the constructor pattern, the class name should be in title case.

Let’s create a new Calculator class.

class Calculator {
	// ...

Inside a class, the constructor function is always named constructor() (with a lowercase “c”).

We can copy/paste the code from the Constructor() function in our constructor pattern library into our class.

class Calculator {
	 * Create the Constructor object
	 * @param {Number} num     The starting total
	 * @param {Object} options Options and settings
	constructor (num = 0, options = {}) {

		// Combine user options with defaults
		let {max, min} = Object.assign({
			max: Infinity,
			min: -Infinity
		}, options);

		// Define properties
		Object.defineProperties(this, {
			total: {
				value: num,
				writable: true
			_max: {value: max},
			_min: {value: min}



Now, we have a library that we can instantiate the same way we did with our constructor pattern library.

// Create a new instance of our library
let age = new Calculator(30);

Adding methods to our Class Pattern library

To attach methods to an instance’s prototype, all you have to do is include them inside the class. You can omit the function keyword.

With this approach, every function gets attached to the instance (this). Accordingly, we need to update all of our validate() functions to this.validate().

 * Add two or more numbers together
 * @param {...Number} nums The numbers to add
add (...nums) {

	// Loop through each number and do math
	for (let num of nums) { = + num;

	// Validate the total

	// Return the instance
	return this;


Now, we can run our instance methods just like with our constructor pattern.

// Do some math
// is now 72
age.add(1, 2, 3).multiply(2);

Static properties

In our class pattern, the random() method is now attached to each instance rather than the Calculator object.

// This throw an error
// Uncaught TypeError: Calculator.random is not a function

// This works
let age = new Calculator(30);

In a JavaScript class, you can use the static keyword to mark a property or function as static.

This will attach it directly to the class object rather than to the prototype.

 * Generate a random whole number
 * @return {Number} A random number
static random (max = 100) {
	return Math.floor(Math.random() * max);

Now, we can run the Calculator.random() method just like before.

// This works again

Tomorrow, we’re going to look at one of my favorite things about JS classes: private Class features.