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Possible Duplicate:
Null object in javascript

Null is an object right? So if i set x to null, why can't i get the constructor value? 

 var x = null;
 alert(typeof x);
 alert(x.constructor);
Community
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steve
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2 Answers2

10

null in JS is a primitive value. It wasn't constructed by any constructor function, so it doesn't have a constructor property. typeof null being 'object' is basically a horrible lie retained for historical reasons. Don't expect too much consistency from JS, you'll be disappointed!

Primitive values can often behave like objects in JS due to autoboxing: (1).constructor works despite 1 also being a primitive value, because it implicitly calls new Number(1). But there is no Object form of null (or undefined) to automatically convert to, so in this case you get an error.

In general you should avoid using constructor. For many class/instance models built on top of JS it doesn't do what you think at all. instanceof typically works better.

bobince
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Null is not an object in Javascript - null is the singular instance of the null primitive type, and as such has no constructor.

Dylan Beattie
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  • Yep, but there's only one of it, and only nothing can be nothing - nothing else can be nothing, and if you make anything something it's not nothing anymore. – Dylan Beattie Dec 06 '10 at 20:26