JavaScript Inheritance: Object.Create VS New

JavaScript inheritance: Object.create vs new

In your question you have mentioned that Both examples seem to do the same thing, It's not true at all, because

Your first example

function SomeBaseClass(){...}
SomeBaseClass.prototype = {
    doThis : function(){...},
    doThat : function(){...}
}
function MyClass(){...}
MyClass.prototype = Object.create(SomeBaseClass.prototype);

In this example, you are just inheriting SomeBaseClass' prototype but what if you have a property in your SomeBaseClass like

function SomeBaseClass(){ 
    this.publicProperty='SomeValue'; 
}

and if you use it like

var obj=new MyClass();
console.log(obj.publicProperty); // undefined
​console.log(obj);​

The obj object won't have publicProperty property like in this example.

Your second example

MyClass.prototype = new SomeBaseClass();

It's executing the constructor function, making an instance of SomeBaseClass and inheriting the whole SomeBaseClass object. So, if you use

    var obj=new MyClass();
    console.log(obj.publicProperty); // SomeValue
    console.log(obj);​

In this case its publicProperty property is also available to the obj object like in this example.

Since the Object.create is not available in some old browsers, in that case you can use

if(!Object.create)
{
    Object.create=function(o){
        function F(){}
        F.prototype=o;
        return new F();
    }
}

Above code just adds Object.create function if it's not available so you can use Object.create function and I think the code above describes what Object.create actually does. Hope it'll help in some way.

What is the real difference between new vs Object.create()

1. new can be used only with a function or a class.
   When a function is executed as new User(...), it does the following steps:

   • A new empty object is created and assigned to this.

   • The function body executes. Usually it modifies this, adds new properties to it.

   • If there is no explicit return statement, the value of this is returned.

---

2. b1obj.key1 is undefined because Object.create first parameter is an object wich becomes a prototype of created object. In this case it is the a1 function, which doesn't have a key1 property assigned, it only assignes key1 to its this when called.

---

3. b2obj has the a2 object as its prototype, so it has access to its key2 property

Suming up: while new keyword is more often used to create new instances of objects from an existing 'template', Object.create is more flexible and allows you to work with a prototype, create property descriptors. For example, you can create a shallow copy of object:

let clone = Object.create(Object.getPrototypeOf(obj), 
                          Object.getOwnPropertyDescriptors(obj));

I suggest you to read this article about new keyword and this one about Object.create and other prototype methods.

UPDATE (about the relationship between object and its prototype):
Once you have created a new object, changing its properties does not affect the prototype. For example,

let a2 = {
    key2: "some text"
};
let b2 = Object.create(a2);
a2.key2 = "I am a2";
b2.key2 = "I am b2";

alert(a2.key2 + ", " + b2.key2);

will alert I am a2, I am b2. That's because b2 has its own key2 property. But If it doesn't, JavaScript will look for it in its prototype. You can find a detailed description of the prototype inheritance here

Difference between Object.create(Parent.prototype) vs Object.create(Parent) during prototype inheritance

You usually want to be using Object.create(prototype) for creating an object from another object and Object.create(Class.protoype) for creating an object from a function.

Doing Object.create(Class) will use the function as a template, meaning only the "static" fields will be transferred to the new object. Object.create(Class.protoype) will use the prototype as the template, thus you will be able to get all fields that were declared using Class.prototype.field, you can also get the static fields through the constructor, and finally, once the constructor function is run on the new object the fields declared inside the constructor will also be created.

function Foo() {  // Only visible by prototype after running constructor  this.test = function() {};}Foo.prototype.bar = "Bar"; // Visible by prototypeFoo.fooBar = "FooBar"; // Visible by class
var classFoo = Object.create(Foo);var protoFoo = Object.create(Foo.prototype);
// fooBar is static, so it can be viewd bu classFooconsole.log(classFoo.fooBar); // bar was added to the prototype, cant be viewedconsole.log(classFoo.bar); // test is declared in the constructor, to which there is no pointer from classFooconsole.log(classFoo.test); // this constructor is the base constructor for all classesconsole.log(classFoo.constructor); 
// fooBar was added to the constructor, so it cannot be viewed this wayconsole.log(protoFoo.fooBar); // but it can be viewed like thisconsole.log(protoFoo.constructor.fooBar); // this is the Foo function/constructorconsole.log(protoFoo.constructor); // bar is added to the prototype so it can be viewedconsole.log(protoFoo.bar); // test has not been declared yet as the constructor has not been run yetconsole.log(protoFoo.test);// the foo function is run with protoFoo as thisprotoFoo.constructor();// the test function has now been added to protoFooconsole.log(protoFoo.test);

JavaScript inheritance with Object.create()?

Object.create() is used to inherit objects, not constructors like you're trying to do. It pretty much creates a new object with the old object set as its prototypal parent.

var A = function() { };
A.prototype.x = 10;
A.prototype.say = function() { alert(this.x) };

var a = new A();
a.say(); //alerts 10

var b = Object.create(a);
b.say(); //alerts 10
b.x = 'hello';
b.say(); //alerts 'hello'

And just to make sure b is not just a clone of a,

a.x = 'goodbye';
delete b.x;
b.say(); //alerts 'goodbye'

Understanding the difference between Object.create() and new SomeFunction()

The object used in Object.create actually forms the prototype of the new object, where as in the new Function() form the declared properties/functions do not form the prototype.

Yes, Object.create builds an object that inherits directly from the one passed as its first argument.

With constructor functions, the newly created object inherits from the constructor's prototype, e.g.:

var o = new SomeConstructor();

In the above example, o inherits directly from SomeConstructor.prototype.

There's a difference here, with Object.create you can create an object that doesn't inherit from anything, Object.create(null);, on the other hand, if you set SomeConstructor.prototype = null; the newly created object will inherit from Object.prototype.

You cannot create closures with the Object.create syntax as you would with the functional syntax. This is logical given the lexical (vs block) type scope of JavaScript.

Well, you can create closures, e.g. using property descriptors argument:

var o = Object.create({inherited: 1}, {
  foo: {
    get: (function () { // a closure
      var closured = 'foo';
      return function () {
        return closured+'bar';
      };
    })()
  }
});

o.foo; // "foobar"

Note that I'm talking about the ECMAScript 5th Edition Object.create method, not the Crockford's shim.

The method is starting to be natively implemented on latest browsers, check this compatibility table.

Prototypal inheritance: Object.create vs Object.assign

We achieve the same result

No, we don't. Bar.prototype would then not inherit from Foo.prototype, instead it would have its own properties. Sure, the values from Foo.prototype would be copied over, but that's just a snapshot of Foo.prototype from the time Object.assign was called instead of a live connection.

---

Related Topics