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Why do we use prototypes in JavaScript?

I would like to know why we use prototypes. I understand that each time we create a new instance of a function, it'll create new space in memory. Are all elements attached to this prototype object? Why and when should we use something.prototype.somethingelse?

2 Answers

Steven Parker
Steven Parker
215,939 Points

Basically, prototypes are a way of establishing properties and methods that will be inherited, or adding new properties and methods to an existing object or set of objects of a given type.

Now I realize that probably isn't much help in itself, but I found this article and discussion thread: JavaScript Prototype in Plain Language over at JavaScriptIsSexy.com that might be more enlightening.

jason chan
jason chan
31,008 Points

We use prototypes for inheritance.

Since javascript isn't a formal object oriented language. That's how we do inheritance.

function Person(first, last) {
this.firstname = first;
  this.lastname = last;
  console.log("This function is invoked.");

// inheriting from Person constructor to create a new method getFullName
Person.prototype.getFullName = function() {
 return this.firstname + ' ' + this.lastname; 

var john = new Person('john', 'doe');

var jane = new Person('jane', 'doe');

Person.prototype.getFormalFullName = function(){
return this.lastname + ', ' + this.firstname; 


// emca2015 you have to use babel to compile this to javascript. This more traditional way create classes and inheritance. This also is similar to typescript

"use strict";

// we create a class here a blue print using the keyword constructor and class
class Polygon {
  constructor(height, width) {
    this.height = height;
    this.width = width;

// we extend or inherit from Polygon with new methods. Basically we just adding new functions to Polygon, but we creating a new object called Square.
class Square extends Polygon {
  constructor(sideLength) {
    super(sideLength, sideLength);
  get area() {
    return this.height * this.width;
  set sideLength(newLength) {
    this.height = newLength;
    this.width = newLength;

var square = new Square(2);