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What is o ?

If I am not mistaken, prototype means that F is going to inherit from o , but that is all I can understand here.

Can someone break this down for me:

if (!Object.create) {
  Object.create = (function () {
  var F = function(){};

  return function (o) {
    if (arguments.length !== 1) {
        throw new Error('Object.create implementation only accepts one parameter.');
    }
    F.prototype = o;
    return new F();
  };
}());

What is o ?

If I am not mistaken, prototype means that F is going to inherit from o , but that is all I can understand here.

Can someone break this down for me:

if (!Object.create) {
  Object.create = (function () {
  var F = function(){};

  return function (o) {
    if (arguments.length !== 1) {
        throw new Error('Object.create implementation only accepts one parameter.');
    }
    F.prototype = o;
    return new F();
  };
}());

• javascript

Share Improve this question Follow edited Nov 23, 2016 at 0:13 MrE 20.8k1414 gold badges9191 silver badges107107 bronze badges asked Jul 31, 2013 at 19:13 fish manfish man 10111 silver badge77 bronze badges 9

• There seems to be an error here: F is initialized only once, so every time you call Object.create() you change the prototype of every other object that you created with it. – Russell Zahniser Commented Jul 31, 2013 at 19:19
• @RussellZahniser I've been scratching my head over that too. I can't get that behavior to materialize when I try it however. – Pointy Commented Jul 31, 2013 at 19:22
• It appears that this is the remended polyfill from MDN, so I assume I am just missing something. – Russell Zahniser Commented Jul 31, 2013 at 19:24
• @RussellZahniser the stuff that goes on with new and the prototype etc. is sufficiently bizarre that I almost never program that way :) – Pointy Commented Jul 31, 2013 at 19:26
• @RussellZahniser, The fact that we are reusing the same empty function F as a constructor will not affect in any way the various created instances. Here's what happens: F is a function that has an initial prototype wich is an object that has it's prototype chain set to Object and which has it's constructor property set to F. When doing new F(), it creates a new object that has it's prototype chain set to the prototype of F and it runs the function F in the context of that new object. – plalx Commented Jul 31, 2013 at 19:30

 |  Show 4 more ments

2 Answers 2

Sorted by: Reset to default 4

o is a parameter that you pass into the function.

For example:

var myObj = Object.create({
        myFn: function(){}
});

Then you can do:

myObj.myFn();

The context of that code can help you understand it better.

This polyfill covers the main use case which is creating a new object for which the prototype has been chosen but doesn't take the second argument into account. - https://developer.mozilla/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/create#Polyfill

The code is intended to serve as a replacement for Object.create(proto [, propertiesObject ]) when Object.create is not provided by the environment.

So, o is just the object which will serve as the prototype of the new object you are creating.

In practice, this means that we can use o as our superclass (or more correctly o is the prototype of our superclass).

The example from the Mozilla docs makes this clear:

//subclass extends superclass
Rectangle.prototype = Object.create(Shape.prototype);

---

Answering the ment, this feels a little too long for the ment box:

o is the prototype of the object you want to inherit from. Technically it is an object too, everything in JS is an object. How is almost everything in Javascript an object?.

Why would you want to inherit from another object? What's the point of OOP?

What does this line do? F.prototype = o;

Lets repeat the full code, except with ments:

// if the execution environment doesn't have object.create, e.g. in old IE
if (!Object.create) {
    // make object.create equal to something
    Object.create = (function(){
        // create a new object F which can be inherited from
        function F(){}
        // return a function which 
        return function(o){
            // throw an error if 
            if (arguments.length != 1) {
                throw new Error('Object.create implementation only accepts one parameter.');
            }
            // force F to inherit o's methods and attributes
            F.prototype = o
            // return an instance of F
            return new F()
        }
    })() // this is an IIFE, so the code within is automatically executed and returned to Object.create
}

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