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javascriptinheritance
Roli Agrawal
Roli Agrawal

Reputation: 2466

understanding javascript inheritence

I am learning javascript inheritance so I am trying the following code:

var obj = function(name) {
  this.name=name
};

var obj1 = new obj("suman"); 
console.log(obj1.name); 

var obj2 = new obj1("sonali"); 
console.log(obj2.name);

But when I try getting obj2.name I get the following error object is not a function.

As per my understanding var obj1 is inheriting properties from obj . So when I am inheriting obj2 from obj1 it should inherit name property also. Please guide me where I am wrong.

Upvotes: 1

Views: 143

Answers (2)

Bergi
Bergi

Reputation: 664538

but when I am trying for obj2.name, I am getting error object is not a function.

Not exactly. You're getting that error when you call var obj2=new obj1("sonali"); - where obj1 is the first instance, not your constructor. Use

var obj2 = new obj("sonali");
console.log(obj2.name); // works as expected

For better readability, the convention is to capitalise the names of constructor functions, so function Obj(name){…} and var obj = new Obj(…).

per my understanding obj1 is inheriting properties from obj

No, obj1 is an instance. It does have the .name property on itself, it doesn't inherit it, it does own it. obj is the constructor function that created the property as part of the instance initialisation.

obj1 does inherit properties from obj.prototype - since you didn't create any custom ones, the only inherited property is obj1.constructor.

So when I am inheriting obj2 from obj1 it should inherit name property also.

As stated above, that's not what's happening. obj1 is not a constructor function, you cannot call it with new. I bet I'm confusing you now, but you can still inherit from the obj1 instance object though: by using Object.create. Let's start with

function Obj(name) {
  this.name = name;
}
var obj1 = new Obj("suman"); // or alternatively:
obj1 = Object.create(Obj.prototype); obj1.name = "suman";

console.log(obj1.name); // suman

Now we can create a new object, inheriting prototypically from the first object:

var obj2 = Object.create(obj1);
console.log(obj2.name); // suman - inherited!
console.log(obj2.hasOwnProperty("name")); // false - it's inherited!

But now we can give it an own name as well:

obj2.constructor("sonali"); // which executes the following:
obj2.name = "sonali"; // pretty equivalent

console.log(obj2.name); // sonali

Upvotes: 4

Sarath Chandra
Sarath Chandra

Reputation: 1888

You are confusing the concepts of 'class' and 'instance' which have specific meanings in Object Oriented literature.

In your example, obj refers to the class/protoype or broadly speaking, a "template" that is used as a reference for creating copies. The object reference provided in the statement using the new operator is the 'instance' created for the class. So, obj1 is a valid instance.

So, only classes can be instantiated and not their instances. To answer your question instantiating a class does not lead to the child 'inheriting' the ability to be instantiated further.

Hence, the objects you wish to create should always refer the 'class' you wish to use while calling the constructor with the new keyword.

Upvotes: 2

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