Why use typeof for identifying a function?

Question

Are there any significant reasons for using

typeof variable === \'function\'

versus

!!variable.call

for d

Answer 1

According to the ECMAScript specification, the following should apply for regular expression literals:

A regular expression literal is an input element that is converted to a RegExp object (section 15.10) when it is scanned. The object is created before evaluation of the containing program or function begins.

So typeof /regex/ should yield "object":

typeof /regex/ === "object"

And the constructor of the object created by the regular expression literal should be RegExp:

/regex/.constructor === RegExp

Similar to that, a function definition should yield a Function object:

(function(){}).constructor === Function

But although this returns a Function object, the typeof operator should not yield "object" but "function" instead:

typeof function(){} === "function"

This is due to the distinction whether the object implements the internal [[Call]] property that is special for Function objects.

Note that all this is how Javascript implementations should behave. So all equations are asserted to be true.