Q:

How to test whether `fn` is a function(callable)?

In which condition will Object.prototype.toString.call(fn) === '[object Function]' and typeof fn === 'function' return different results?
I saw the function isCallable on mdn (see line 4 - line 7):
var isCallable = function (fn) {
    var toStr = Object.prototype.toString
    return typeof fn === 'function' || toStr.call(fn) === '[object Function]'
}

I'm wondering the difference between these two tests, is one of them superfluous?

A:

Object.prototype.toString returns the value of the object's internal [[Class]] property, it isn't really a Type.
The value of this internal property represents the specification defined classification of an object (more info here).
The value of the [[Class]] internal property for host objects -as DOM elements- can be anything, it is completely implementation-dependent.
So, the best way to test whether fn is a function is to use typeof fn.
Not only is it faster but the ECMAScript specification ensures that all functions have a type of "function" and that only functions can have a type of "function" (see also https://stackoverflow.com/a/17108198/709439).

