[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.

[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".

[firefoxman1]

Oh yeah, of course. There's no need when you can just do === null.