CODEWITHSUNDEEP
javascript
aztack
aztack

Reputation: 4594

Is undefined a property of window/global?

It seems that the undefined is a property of window/global :

enter image description here

I always thought that undefined is, like null, a uniqe value in JavaScript.

But above code (tested in Chrome) make me confused.

Can some explain why

undefined in window

evalute to true, while

null in window

evaluate to false

Upvotes: 11

Views: 1493

Answers (1)

Nathan
Nathan

Reputation: 4067

Not only undefined, but also Infinity and NaN are values of the global object, in this case, window (as of ES5.1 specification).

The fact you can't assign a value to undefined is because the property is defined with the writable attribute set to false.

null is a primitive value (as is 5) of the type Null (as is Number for 5), not a property of window.

Take a look at the annotated ES5 specification for more background on this, its quite readable!

Upvotes: 13

Related Questions