CODEWITHSUNDEEP
javascriptinheritanceprototypeprototypal-inheritanceprototype-chain
Robert
Robert

Reputation: 10390

JavaScript iterating over object properties and the prototype chain

MDN states:

Also, when iterating over the properties of an object, every enumerable property that is on the prototype chain will be enumerated.

So I tried this:

var x = {a: "I am a"};
var z = Object.create(x);

for( i in z )
{
    console.dir( i );

    if( i == "hasOwnProperty" ) {
        console.log( 'found hasOwnProperty' );
    }
}

Outputs only a but not hasOwnProperty. Why?

Upvotes: 2

Views: 259

Answers (3)

Lampent
Lampent

Reputation: 1

As answered above, each property of an object has an 'enumerable' flag. When the flag set to false the property will not be enumerated when iterating over object properties.

Object.prototype.hasOwnProperty is non-enumerable which mean is 'enumerable' flag set to false.

You can read an article I wrote on the topic here to deepen your knowledge.

Upvotes: 0

KarimS
KarimS

Reputation: 3892

Because hasOwnProperty is not enumerable , you can test it using 

console.log(Object.getOwnPropertyDescriptor(Object.prototype, "hasOwnProperty").enumerable)

Upvotes: 1

Oriol
Oriol

Reputation: 288100

Because Object.prototype.hasOwnProperty is non-enumerable:

Object.getOwnPropertyDescriptor(Object.prototype, 'hasOwnProperty')
  .enumerable // false

Therefore, it's not iterated by the for...in loop.

Upvotes: 8

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