Using JavaScript classes without the `class` keyword

June 29, 2023

1,302 words

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Classes in JavaScript are both powerful and weird. While they allow us to create named objects with similarly purposed methods and properties, they're often misunderstood because of nuanced in the language itself.

But did you know that prior to 2015, JavaScript didn't even have a class keyword as part of the language?

Despite this, many programs at the time used classic Object Oriented Programming (OOP) methodologies such as using a class, extending it, and even adding static methods.

But without a class method, how did they even make classes?

A good question! Let's answer that and, along the way, look at:

  • How to create a "class" without the class keyword
  • How to "extend" a "class"
  • How to add static methods to our "class"

Create public fields with the constructor

Let's look at a modern JavaScript class:

javascript
class User {
name = "Corbin",
username = "crutchcorn",
sayCatchphrase() {
console.log("It depends");
}
}

This is a fairly basic class that has two properties (name and username) as well as a sayCatchphrase method.

However, despite the class keyword being added in 2015 with ES6, public fields like this weren't added until ECMAScript 2020:

A JavaScript compatibility table showing support for class added in Node 6, but "Public fields" added in Node 12

So then how did classes get properties in years after 2015 but before 2020?

The answer? The constructor method:

javascript
class User {
constructor() {
this.name = "Corbin",
this.username = "crutchcorn",
}
sayCatchphrase() {
console.log("It depends");
}
}

In fact, using this constructor method, we can even add the method as well:

javascript
class User {
constructor() {
this.name = "Corbin",
this.username = "crutchcorn",
this.sayCatchphrase = function() {
console.log("It depends");
}
}
}

An interesting fact, for sure - but it doesn't answer the question of how to make a class.

Don't worry, we're getting there!

Create a class without the class keyword

Before we answer the question of "how to make a class in JavaScript without the class keyword", let's take a step back and look at what a class is actually doing...

After all, a class like User above might create an object like so:

javascript
const userObject = {
name: "Corbin",
username: "crutchcorn",
sayCatchphrase: function() {
console.log("It depends");
}
}

Knowing this, we might think that the best way to make a class without the keyword is to return an object from a function:

javascript
function User() {
return {
name: "Corbin",
username: "crutchcorn",
sayCatchphrase: function() {
console.log("It depends");
}
}
}

And sure enough, if we run this code using:

javascript
const user = new User();
user.sayCatchphrase(); // "It depends"

It will run as-expected. However, it won't solve all cases. EG:

javascript
new User() instanceof User; // false

Instead, what if we just converted the aforementioned class' constructor body to a function?:

javascript
function User() {
this.name = "Corbin";
this.username = "crutchcorn";
this.sayCatchphrase = function() {
console.log("It depends");
}
}

Now, not only do we have the method working, but instanceof works as well:

javascript
const user = new User();
user.sayCatchphrase(); // "It depends"
new User() instanceof User; // true

Prototype Manipulation

But surely changing from a class to a function doesn't allow you to change the prototype in the same way?

Actually, it does! That's how this whole thing works!

Consider the following code:

javascript
function User() {
this.name = "Corbin";
this.username = "crutchcorn";
}
User.prototype.sayCatchphrase = function() {
console.log("It depends");
}

This is the same way of adding a method as the this.sayCatchphrase method as before, but is done by changing the prototype.

We can test this code still works by running:

javascript
const user = new User();
user.sayCatchphrase(); // "It depends"

Create an extended class using the super method

Before we talk about function-based class extension, we need to talk about pre-ES2020 class creation once again.

See, when we convert the following code to use a constructor:

javascript
class Person {
personality = "quirky";
}
class Corbin extends Person {
name = "Corbin";
}

Like so:

javascript
class Person {
constructor() {
this.personality = "quirky";
}
}
class Corbin extends Person {
constructor() {
this.name = "Corbin";
}
}

And try to initialize it:

javascript
const corn = new Corbin()

We get the following error:

Uncaught ReferenceError: Must call super constructor in derived class before accessing 'this' or returning from derived constructor
at new Corbin (<anonymous>:9:6)

This is because we're not using the super() method to tell our extended class to utilize the parent's class' methods.

To fix this, we'll add that method to the extended class' constructor:

javascript
class Person {
constructor() {
this.personality = "quirky";
}
}
class Corbin extends Person {
constructor() {
super();
this.name = "Corbin";
}
}

Now our Corbin constructor work work as-intended:

javascript
const corn = new Corbin();
console.log(corn.name); // "Corbin";
console.log(corn.personality); // "quirky";

Extend a functional class using Object.create

Let's now convert our Person and Corbin classes to use functions instead of the class keyword.

The person class is easy enough:

javascript
function Person() {
this.personality = "quirky";
}

And we could use the call method to bind Person's this to Corbin, like so:

javascript
function Corbin() {
Person.call(this);
this.name = "Corbin";
}

And it appears to work at first:

javascript
const corn = new Corbin();
console.log(corn.name); // "Corbin";
console.log(corn.personality); // "quirky";

But now, once again, if we call instanceof it doesn't support the base class:

javascript
new Corbin() instanceof Corbin; // true
new Corbin() instanceof Person; // false

To fix this, we need to tell JavaScript to use the prototype of Person and combine it with the prototype of Corbin, like so:

javascript
function Person() {
}
Person.prototype.personality = "quirky";
function Corbin() {
}
Corbin.prototype = Object.create(Person.prototype);
Corbin.prototype.name = "Corbin";
const corn = new Corbin();
corn.personality // "quirky"
corn.name // "Corbin"
const pers = new Person();
pers.personality // "quirky"
pers.name // undefined

Notice how we're using Object.create to create a base object from the other prototype

Static Methods

Let's wrap up this article by talking about how to add static methods to a functional class.

As a refresher, this is what a static method looks like on a ES2020 class:

javascript
class User {
name = "Corbin",
username = "crutchcorn",
static sayCatchphrase() {
console.log("It depends");
}
}
User.sayCatchphrase(); // "It depends"
User.name // undefined
const corn = new User();
corn.name; // "Corbin"

This can be added by providing a key to the function's name outside of the function body:

javascript
function User() {
this.name = "Corbin",
this.username = "crutchcorn",
}
User.sayCatchphrase() {
console.log("It depends");
}
User.sayCatchphrase(); // "It depends"
User.name // undefined
const corn = new User();
corn.name; // "Corbin"

Conclusion

This has been an interesting look into how to use JavaScript classes without the class keyword.

Hopefully, this has helped dispel some misunderstandings about how classes work in JavaScript or maybe just given historical context for why some code is written how it is.

Like learning JavaScript's fundamentals?

Check out my article that explains how to use the .bind keyword in JavaScript.

Read it and want more?

Check out my book that teaches the introduction of React, Angular, and Vue all at once; "The Framework Field Guide".

Until next time!

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