CODEWITHSUNDEEP
javascript
AL-zami
AL-zami

Reputation: 9066

does inheriting from another constructor overridedes the existing properties from the prototype chain of the constructor that inherits

Here i have two constructor say First and Second.

First inherits from Second.First also has a hair property on its prototype chain.

A newly created object say tom which supposed to be an instance of First constructor has also a user-defined property called nail.

If i want to log all the enumerable properties of tom object using a for...in loop it only shows name,nail and age.But hair property seems to be disappeared from its prototype chain.

Why hair property disappeared from the prototype chain?? How can i get it back??

<html>
<body>
<script>
   function First(name){
       this.name=name;
   }
   First.prototype.hair='black';// tom.hair gets me undefined
   function Second(){
       this.age=1;
   }

   First.prototype=new Second();
   var tom=new First('tom');
   tom.nail='sharp';// added property to tom
   for(var i in tom){
       console.log(i);
   }
   console.log(tom.hair);
</script>
</body>
</html>

Upvotes: 0

Views: 39

Answers (3)

loveNoHate
loveNoHate

Reputation: 1547

You overrode the prototype on

First.prototype=new Second();

make it 

First.prototype.age=new Second().age;

and it works.

JSFIDDLE.

Upvotes: 2

CupawnTae
CupawnTae

Reputation: 14580

The problem is you set the hair property on the default prototype for First and then replace it with a new prototype, which means your original is lost. If you reorder the statements like this it will work:

function First(name){
    this.name=name;
}
function Second(){
    this.age=1;
}

First.prototype=new Second();
First.prototype.hair='black'; // this works now!

var tom=new First('tom');
tom.nail='sharp';// added property to tom
for(var i in tom){
    console.log(i);
}
console.log(tom.hair);

Upvotes: 1

Russ Cam
Russ Cam

Reputation: 125498

First.prototype.hair

is declared before 

First.prototype

is overwritten with an instance of an object constructed by Second when 

First.prototype=new Second();

is called. To keep the hair property added to the prototype of First, add it to the prototype after you assign a instance of an object created by calling Second i.e.

function First(name){
    this.name=name;
}

function Second(){
    this.age=1;
}

First.prototype=new Second();

// add to prototype after overwriting/assigning prototype above
First.prototype.hair='black';

var tom=new First('tom');
tom.nail='sharp';

for(var i in tom){
    console.log(i);
}

Upvotes: 1

Related Questions