What is prototype in JavaScript?

The entire inheritance concept in JavaScript is based on prototype, prototype inheritence is whenever we create any object or function in JavaScript it automatically inherit properties and methods from certain template object or prototype object.

Here you can understand better, there are number of prototype objects in JavaScript (eg: Array.prototype, Function.prototype, Object.prototype etc.). So whenever we create any array, function or object JavaScript engine internally attaches the prototype template's properties and methods into your newly created object. that is why we can access in-built functionality of certain objects like when we create any array we get method like push, map, similarly for functions we get call, apply and bind and for objects we get toString, hasOwnProperty and etc.

In JavaScript everything is object, and every object has a property as __proto__, in this property you can see all properties and methods that acquired from another object or template object.

let object = {};


// constructor: ƒ Object()
// hasOwnProperty: ƒ hasOwnProperty()
// isPrototypeOf: ƒ isPrototypeOf()
// propertyIsEnumerable: ƒ propertyIsEnumerable()
// toLocaleString: ƒ toLocaleString()
// toString: ƒ toString()
// valueOf: ƒ valueOf()
// __defineGetter__: ƒ __defineGetter__()
// __defineSetter__: ƒ __defineSetter__()
// __lookupGetter__: ƒ __lookupGetter__()
// __lookupSetter__: ƒ __lookupSetter__()
// get __proto__: ƒ __proto__()
// set __proto__: ƒ __proto__()

The output you are seeing here will be the same as Object.prototype.

What is __proto__?

The __proto__ attribute used to assign the properties and methods from prototype template. Whenver we want to inherit propeties and methods from prototype template we can just assign it into child's __proto__ attribute, You can directly understand this field by below example:

const human = {
  teeth: 32,

const john = {
  __proto__: human,
  leg: 2,

// 32

So, here in the above example we've created a human object and it have a field as teeth with value 32, and we've also created another object as john, and it have 2 fields first is field leg and it have value as 2, but here we want to inherit properties from human object in john object, so we've just assigned human into john's __proto__.

Let's see how it printing the value.. whenever we access john.teeth, javascript engine will try to find the teeth field inside john object and when it didn't find, it will go to john's __proto__ and if teeth available, it will print the value.

So whenver the finding goes on various level, known to be as Prototypical Chain.

We've already seen the example, how to use the prototype property, how can we make available other properties and methods for newly created objects in the article Polyfill for bind()-step-by-step.


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