

JavaScript - "typeof" operator & null - krisk
http://www.kiro.me/blog/typeof_null.html

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yahelc
Another fun typeof quirk that the table leaves out:

Regular Expressions, depending on the V8 or Nitro engine version, are
implemented to return either 'object' or 'function'
<http://bonsaiden.github.com/JavaScript-Garden/#types.typeof>

Recent versions of V8 and Nitro return 'object' now, but presumably, there are
many people still running browsers that return 'function'.

Seems like it relies on whether or not the RegExp is callable, but that the 2
engines removed 'function' as they ceased supporting callable RegExes.
<http://code.google.com/p/v8/issues/detail?id=617>

~~~
firefoxman1
I often use the improved typeof function:
[http://javascriptweblog.wordpress.com/2011/08/08/fixing-
the-...](http://javascriptweblog.wordpress.com/2011/08/08/fixing-the-
javascript-typeof-operator/)

Question: why is the way to determine null considered "very bad"? It works and
seems like a valid way, since both empty objects and arrays are truthy, null
is the only falsy "object" type.

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sjs
I think it's only bad because it would break with future changes to typeof. Of
course I may be completely wrong on that.

This works just as well and is more future proof though:

    
    
        function isNull(x) { return x === null }
    

At which point you don't really need a function at all. Perhaps the version
using typeof is from a time when there was no === operator. Was that pre-ES3?
I think so but am not certain.

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krisk
"I think it's only bad because it would break with future changes to typeof"

You're completely right.

The proposal (<http://wiki.ecmascript.org/doku.php?id=proposals:typeof>)
suggests the change of typeof null to actually return "null"; however, as
Douglas Crockford points out, "existing code cannot anticipate the change".
Additionally, Brendan Eich states that "neither this proposal nor the bug
fixes in that proposal are backward-compatible".

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pcwalton
The other gotcha with "typeof": "typeof NaN" is "number", amusingly enough.

~~~
krisk
Indeed. That's because ECMAScript standard states that Numbers should be
IEEE-754 floating point data, which includes positive and negative infinity,
and also NaN. Hence why, in JS, it's part of the Number object: Number.NaN.
Technically, it's stil a numeric data type, but just points to an invalid
number :)

~~~
krisk
I've written another post which explores NaN in detail: "NaN and typeof" -
<http://kiro.me/blog/typeof_nan.html>

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Ralith
You may want to mention the language that you are talking about in future
titles.

~~~
krisk
True. Thanks!

~~~
krisk
There, updated.

