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What is __proto__? | JavaScript

inambe profile image Inam Ul Huq Updated on ・2 min read

You'd probably notice this __proto__ property every time you log an object into the developer console.

proto

In this post, I'm gonna try to explain where this __proto__ comes from, what it contains, and anything in between. So, let's get this __proto__.

First, let's understand how a JavaScript object gets constructed. Once you understand how an object gets constructed, __proto__ property is gonna make a lot more sense than otherwise.

How an object get constructed?

A JavaScript object, always, get constructed/produced by a constructor function. Now, you'd probably say "But, object literals? They are not constructor functions."

Well, object literals are just syntactic sugar. You can write objects without using object literals. See the example below, both syntax would produce the same output. The second approach gives us more power(which we won't discuss here) which object literal takes away from us.

// an object using object literal
const person = {
  name: "John",
  age: 30
};

// an object using Object constructor function
const person = new Object();
Object.defineProperties(person, {
  name: {
    value: "John"
  },
  age: {
    value: 30
  }
});

Now that we know that every object, in JavaScript, is constructed by a constructor function, let's get back to the original question.

What is __proto__?

__proto__ is a property, in every JavaScript object, which points to object's constructor function's prototype property. I know it's a bit hard to grasp, but let me simplify it for you.

Every constructor function has this prototype property on them. The picture below shows us Object constructor function's prototype property.

prototype

So, every object, we'd construct/produce from Object constructor function, would have __proto__ property pointing to Object constructor function's prototype property.

Example

When we use object literals, as we did in the example below, Object constructor function is used to construct/produce the object. Now that we have a person object constructed/produced from Object constructor function, we can verify that __proto__ from our person object is same as prototype from Object constructor function.

// an object using object literal
const person = {
  name: "John",
  age: 30
};
// verify
console.log(person.__proto__ === Object.prototype); // true

If you didn't understand something from the post, or I missed something, please, let me know.

This is my first ever blog post and I'm open to criticism/suggestions.

Posted on Jun 7 '19 by:

inambe profile

Inam Ul Huq

@inambe

Hi, I'm a full-stack developer. I use JavaScript (Node, React), PHP(WordPress), and Flutter. Love Linux and open-source software. 21.

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