Question

Are there any significant reasons for using

typeof variable === 'function'

versus

!!variable.call

for detecting if a variable is a function?

Other than the obvious one that someone may create an object like:

{ call: 1 }

The problem that I have is that

typeof /regex/ === 'function'

returns true, but

!!/regex/.call

returns false

Answer 1

The safest way is to check the internal [[Class]] property by setting the object as the thisArg argument of the .call() method when calling Object.prototype.toString.

Object.prototype.toString.call( myVariable ) === '[object Function]';

Of course you could easily make a function out of it:

function checkClass( obj ) {
    return Object.prototype.toString.call( obj ).slice( 8, -1).toLowerCase();
}

checkClass( myVariable ) === 'function';

This is very simple, and there could be some improvements, but you get the idea.