prototype, __proto__ and Prototypal inheritance in JavaScript

This post was originally published on my website

If you have spent some time with JavaScript, chances are that you have already seen or atleast heard about prototypes. If you are unsure about prototypes or what does it do, this article is just for you. In this article, I will try to help you understand working of JavaScript prototypes and in the process, try to explain what is prototypical inheritance and how does it work.

Before beginning, I hope that you are already aware that everything in JavaScript is a high level object. What this means is except null and undefined, everything in JavaSscript is derived from Object.

But Varun, how is that possible? We have several primitives in JavaScript like String, Number, Boolean etc. How can a data type, say Number, be derived from Object?

prototype and __proto__

To answer that, we need to first understand what is prototype. Prototypes in JavaScript is nothing but a special set of properties which an object holds (remember, almost everything in JavaScript is derived from Object). Each object holds it's own set of prototype properties. Let's see a very basic example of what I mean by that. Open your browser dev tools and try the snippets as you go along.

var fooFunc = function() {
    return {
        foo: 42
    }
};
fooFunc.prototype.bar = 'baz';
var fooVal = fooFunc();
console.log(fooVal);   // {foo: 42}
console.log(fooFunc.prototype);     // {bar: "baz", constructor: ƒ}

The second print statement gives you the example of prototypal inheritance in all it's beauty. Function fooFunc is derived from Object instance and has it's own set of properties with it i.e. {bar: baz} along with whatever it carried along when it instantiated from Object i.e. {constructor: ƒ}.

So if fooFunc is derived from Object, can I go up the chain and see Object's prototype?

Good question and absolutely you can. However one thing you need to keep in mind is that except JavaScript function type, every other prototype of an object resides in it's __proto__ property. Let's see what I mean by that.

console.log('prototype of fooFunc:');
console.log(fooFunc.prototype);     // {bar: "baz", constructor: ƒ}
console.log('prototype of Object:');
console.log(fooFunc.prototype.__proto__);   // {constructor: ƒ, __defineGetter__: ƒ, __defineSetter__: ƒ, hasOwnProperty: ƒ, __lookupGetter__: ƒ, …}

Do you see what I see? The last console statement returns an object with it's own set of special properties. This is nothing but prototype chain of Object. This confirms that we can actually traverse up the prototype chain and that our function fooFunc is derived from Object.

So what happens when I try to traverse the prototype chain of Object?

Let's see what happens:

console.log(fooFunc.prototype); // {bar: "baz", constructor: ƒ}
console.log(fooFunc.prototype.__proto__);// {constructor: ƒ, __defineSetter__: ƒ, …}
console.log(fooFunc.prototype.__proto__.__proto__);     // null

You see, Object in JavaScript is the top level construct. If you try to see what properties does Object's parent hold, you'll get null because there is no parent of Object.

At this point, I would like you to go back to the begining and co-relate everything till here to what I said earlier in the post.

Prototpyes in JavaScript are nothing but a special set of properties held by an object.

Prototypal inheritance

Now that you've understood how prototype works, prototypical inheritance should be pretty straight forward. Let's look at the following example:

var obj = function(){
    this.firstName = 'Varun';
    this.lastName = 'Dey'
}
obj.prototype.age = 25;
var nameObj = new obj()
console.log(nameObj.age);   // 25

Let's break down what is happening over here:

• First off, we are defining a function obj.
• Now we are also assigning another property age directly on obj's prototype chain.
• We instantiate a variable called nameObj from obj. nameObj is an object which gets two properties appended to it namely firstName and lastName.
• When I ask newObj for it's age property, it firstly goes into it's own object and tries to find it. Does it find age in nameObj object?
  • No. So it goes up the chain, which is nameObj.__proto__ and looks for an age property in that object.
  • It finds an age property over here because nameObj.__proto__ is exactly the same as obj.prototype.

And this is what JavaScript's prototypal inheritance is all about. Whenever you ask JavaScript to fetch you a key, it first looks into it's own object's property. If it does not find anything, it goes up to its prototypal chain (obj.__proto__) and tries to find that key among those properties, if it does not find it there, it goes one level up it's current prototypal chain (obj.__proto__.__proto__) and does the same thing. It keeps on repeating the same process until it reaches the Object's prototype chain and returns undefined from there if it can not find it even there.

Prototype pollution

This makes an interesting case of inheritance in JavaScript which is quite different than other class-based languages like Java/C++:

function parent(){
    return{
        foo: 42,
        bar: 'baz'
    }
}
child = new parent()

If you look closely, you will see that child is an instantiated object of parent. And parent ultimately is nothing but an instantiated method of Object. What this means is that child's' and parent's prototype's prototype is Object's prototype

child.__proto__ === parent.prototype.__proto__      // true

Now let's see one more example:

function parent(){
    return{
        foo: 42,
        bar: 'baz'
    }
}
parent.prototype.__proto__.baz = 'I should not belong here'
child = new parent()
console.log(child.__proto__)

Here you see a prime example of protoype pollution. I created a property baz directly on Object's prototype by going over function's prototype chain. Now this baz will be shared across all instances of Object and that is why if you see the console statement, you will find that along with other Object properties, we now also have baz: "I should not belong here". This is a bad practice and is frowned upon as it breaks encapsulation.

Similarly I can also do this and JavaScript would allow me to do so:

function parent(){
    return{
        foo: 42,
        bar: 'baz'
    }
}
delete parent.prototype.constructor
child = new parent()

Performance

Needless to say, as you traverse up your prorototype chain, the lookup time increases and hence the performance suffer. This becomes critical when you are trying to access a non existent property across the full prototypal chain. To check whether the property you need is defined in the object itself, you can use hasOwnProperty.

child.hasOwnProperty('foo');    // true
parent.hasOwnProperty('baz');   // false
Object.prototype.hasOwnProperty('baz'); // true

Completing the circle

In the very beginning, I said that except null and undefined, everything is Object instantiation. Let's prove that:

const foo = 42;
const bar = 'fooBar';
const baz = true;
foo.__proto__.__proto__ === bar.__proto__.__proto__;    // true
bar.__proto__.__proto__ === baz.__proto__.__proto__;    // true

So you see what I'm talking about. Almost everything in JavaScript comes from Object

Conclusion

Prototypes makes the fundamental blocks of JavaScript. I hope I was able to help you understand the how prototypes work in JavaScript. Once you get the proper hang of it, you can extend this knowledge to understand how this works in JavaScript. Mozilla has an excellent resource on this and I encourage you to go through it as well - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Inheritance_and_the_prototype_chain

I would love to hear if this article helped you in understanding JavaScript better. :)