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  • The type of null

The type of null

# Xavier MONTILLET (14 years ago)

I saw in the spec that null is a litteral but typeof null must return 'object'. So I was wondering: Is Object.isExtensible(null) supposed to trow an error or to return false?

Cheers

# David Bruant (14 years ago)

Le 04/09/2011 14:31, Xavier MONTILLET a écrit :

Hi,

I saw in the spec that null is a litteral but typeof null must return 'object'.

There are two things that are different. One is the Null type as defined in ES5.1 section 8.2 and the return value of "typeof null" ('object').

This is a notorious spec bug and is very likely to be fixed in the next version of ECMAScript (see harmony:typeof_null )

So I was wondering: Is Object.isExtensible(null) supposed to trow an error or to return false?

Step 1 of Object.isExtensible is "If Type(O) is not Object throw a TypeError exception.". Here, "Type(O)" refers to the type defined in ES5.1 section 8. Consequently, Type(null) is Null (8.2) which is not Object: "Object.isExtensible(null)" is supposed to throw a TypeError (and does properly in Firefox 6 as I have just tested).

Another notable difference between ECMAScript types and typeof is functions. There is no ECMAScript Function type. Functions are Objects with an internal [[Call]] property (see ES5.1 - 11.4.3 table 20) and for them, the typeof operator returns "function".