Linking Object or Inheriting From Objects

Linking objects is key to understanding the prototype system as it leads up to complex patterns involving constructors and subclasses

Meher Howji avatar image
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In JS, you can link objects and share properties pretty easily. Let's create an object talk that has a function greet. And create 2 objects from where we want to point to the talk object so that we can invoke the greet function as if it exists on the friend or buddy object.

var talk = {
  greet: function () {
    return 'Hello! I am ' +
var friend = {
  name: 'meher',
var buddy = {
  name: 'you',

Now, if you log the friend object, you would see the internal slot called [[Prototype]](in the case of Chrome) and <prototype>(in the case of Firefox). By default, this internal slot <prototype> points to the Object constructor, which is one of the fundamental objectsExternal link icon. In simple terms, fundamental objects are just built-in objects, they are Object, Function, Boolean, and Symbol functions.

For us to make our object invoke the greet function as it was its own, we need to make this internal prototype([[prototype]] or <prototype>) point to the talk object. Because this internal prototype is used for lookup when a property/method doesn’t exist on an object.

Object {
  name: "meher"
  // we want to point this to the `talk` object, as this is used for the lookup
  [[prototype]]: Object {...}

The following 2 ways we can make the internal prototype point to the talk object but we DON’T use them because it has performance implicationsExternal link icon. In a gist, changing the [[prototype]] is generally slow due to how the JS engine optimizes property accesses.

// Try both nonetheless to just see the effect, AVOID using them though
friend.__proto__ = talk
// or
Object.setPrototypeOf(friend, talk)

The correct way to do it would be using the Object.create method. Object.create creates a temporary object that points to the talk object, and we make our object point to this temporary object. Look at this simplified polyfill for Object.create or the one at MDNExternal link icon

Notice how the new keyword does the heavy lifting, take a look at this videoExternal link icon to quickly revise and follow what exactly the new keyword does.

function ObjectCreate(pointTo) {
  function F()
  F.prototype = pointTo
  var obj = new F()
  return obj

Now, coming back to our code, if you couldn't follow the polyfill for Object.create then simply assume it to be a nice little black box that links objects without having a performance implication. And there are 2 ways to use it:

without property descriptor
var friend = Object.create(talk) = 'meher'

Object.create has 2 parameters, the second one is a property-descriptorExternal link icon, which is a fancy word for describing the nature of the properties of your object.

with property descriptor
var friend = Object.create(talk, {
  name: {
    value: 'meher',
    writable: true, // we want the object to be ...