Of Classes and Constructor Functions: How JavaScript is Different from Other OOP Languages

June 19, 2020

A question was raised about the difference between functions and constructor functions in JavaScript. The question follows JavaScript’s notorious reputation of not being a real Object Oriented language.

And while this is true (which we will get into later), popular literature mostly explains why in comparison with traditional OOP languages like C++, Java, or Python. Not only is this not helpful, it’s also confusing for those who aren’t familiar with those languages.

So in this article, I will try to clear up how JavaScript classes are different from traditional OOP classes. I will be using Python as a representative of those languages because it’s easy to understand and its relatively close to JavaScript.

Traditional OOP Languages

A class is often defined as a blueprint for for objects. It serves two practical purposes:

  • Abstraction: which information is relevant? Which is irrelevant?
  • Encapsulation: how do I show or hide what is relevant or irrelevant?

At its very core, a class has two types of properties: members and methods. These properties define the data stored in the class and what operations the class can do on that data.

To make use of a class, we create instances of the class through a process called instantiation. Each instance gets isolated copies of the members and methods of the class. Let’s see how this works in Python:

class Person:
  def __init__(self, first_name, last_name):
    self.first_name = first_name
    self.last_name = last_name
  
  def print_full_name(self):
    print(f'{self.first_name} {self.last_name}')

person_a = Person('Adrian', 'Perea')
person_b = Person('Ben', 'Halpern')

person_a.print_full_name() # Adrian Perea
person_b.print_full_name() # Ben Halpern

In this example, person_a and person_b are instances of Person. Each of them gets their own first_name and last_name members, and their own print_full_name method.

Now in Python, you perform instantiation by just calling the class directly (like how we created person_a and person_b). Traditionally however, this wasn’t always the case. In C++ and Java, for example, you need to add the keyword new in order to be able to instantiate the class. I believe that this is where the confusion starts.

JavaScript

In JavaScript, we have something called constructor functions that we called with the new keyword. These constructor functions are the JavaScript analog of the class. Now while it seems that this is the same thing as the other languages we’ve mentioned, JavaScript behaves differently whenever we use these constructor functions. See, whenever we use the new keyword to execute a constructor function, we’re essentially telling JavaScript to run the function normally, but with two extra steps behind the scenes:

  1. An implicit object is created at the start of the function that we can reference with this.
  2. The resulting instance has a copy of the constructor function’s prototype property inside its own prototype.

Don’t worry about the details for now as we’ll get to those later. Let’s see first how we can make a JavaScript object without any fancy constructor functions:

function Person(firstName, lastName) {
  return {
    firstName,
    lastName,
    fullName() {
      console.log(`${this.firstName} ${this.lastName}`)
    }
  };
}

const personA = Person('Adrian', 'Perea');
const personB = Person('Ben', 'Halpern');

personA.fullName() // Adrian Perea
personB.fullName() // Ben Halpern

This works fully well! Why not call it a day and be done with it?

Well, the brutally honest truth is, we can. There are a lot of things that we can accomplish by simply creating objects this way. But in so doing, we’re missing the entire point of JavaScript being what we call a prototype-based language. This is what makes it unique (not necessarily better nor worse) from the traditional OOP languages.

Now let’s see how we can implement this another way. While you’re reading the following snippet, remember the extra two steps that happen behind the scenes when constructor functions are called with new.

function Person(firstName, lastName) {
  // 1. An implicit object is created that we can reference with `this`
  this.firstName = firstName;
  this.lastName = lastName;
}

// 2. The resulting instance has a copy of the 
// constructor function's prototype property 
// inside its own prototype. 
Person.prototype.fullName = function() {
  console.log(`${firstName} ${lastName}`);
}

const personA = new Person('Adrian', 'Perea');
const personB = new Person('Ben', 'Halpern');

personA.fullName() // Adrian Perea
personB.fullName() // Ben Halpern

Now this is where the magic happens. As you can see, when we created the Person class, we separated where we defined the members (firstName and lastName) and where we defined the method (fullName). firstName and lastName are right where you expect them: inside the constructor function definition. But the interesting part is where we define fullName and that’s in the prototype of the constructor function.

Why is this important? It’s important because whenever we create a new instance of the Person constructor function through the new keyword, a reference to the prototype property of constructor function gets added to the __proto__ property of the object. Read that again. After that, read it one more time. This part is important.

personA.__proto__ === Person.prototype;

As opposed to traditional OOP languages, methods are not copied to each instance of the constructor function (or class). When we call personA.fullName(), instead of finding the method in the instance itself, JavaScript looks at the __proto__ property of personA and climbs up until it finds fullName. Since we defined fullName in Person.prototype, and since Person.prototype is the same as personA.__proto__, when we call personA.fullName(), we’re calling a method that exists not in the instance but in the constructor function itself! This provides performance benefits since the methods only have to be defined once (on the prototype of the constructor function). That’s to say:

personA.fullName === personB.fullName === Person.prototype.fullName;

This means that whatever we define on Person.prototype will be available to all instances of Person. In effect, we can do something weird (in traditional OOP sense) like this:

Person.prototype.sayHi = function() {
  console.log(`Hi! I'm ${this.firstName}`);
}

// Note that we did not recreate the objects here
personA.sayHi(); // Hi! I'm Adrian
personB.sayHi(); // Hi! I'm Ben

So there you have it. To sum things up:

  • Constructor functions do two things in the background whenever they are called with new: create an implicit object that can be referenced with this, and assign the __proto__ property of each instance to refer to the prototype property of the constructor function
  • When a function is called on the instance, the __proto__ property is climbed until a reference to the called function is found. This means that each instance doesn’t have a reference to the method, but all share the same method that’s defined on the constructor function.
  • In traditional OOP, all instances have a copy of each method. There is no concept of prototypes.

What about ES6 “classes”

ES6 “classes” don’t really introduce the classes as we traditionally know them. It makes writing constructor functions easier since you wouldn’t have to write prototype for each method you want to share amongst instances. ES6 class syntax is simply an easier way to store all members and methods of a constructor function all in one place, while also abstracting prototype and all the confusion it brings.

As an example, we can write the Person constructor function the following way:

class Person {
  constructor(firstName, lastName) {
    this.firstName = firstName;
    this.lastName = lastName;
  }

  fullName() {
    console.log(`${firstName} ${lastName}`);
  }
}

You can see that it looks very similar to our python example (but you and I both know they’re not the same!). Try creating instances of the Person and look at the prototype property yourself! 😉

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Hi! I'm Adrian. I'm a software engineer, and I work hard to provide helpful and highly intuitive content for free. If you like what you read, please consider following me on Twitter. Hope to see you again next time!

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