TypeError: Function.prototype.toString is not generic

[Xotic750]

I was performing some tests on a module which was converted to use isCallable instead of typeof and I came across this error.

TypeError: Function.prototype.toString is not generic

Following through the test code, this is what causes the error to be thrown

function NameBuilder(first, last) {
    this.first = first;
    this.last = last;
}

NameBuilder.prototype = Object;

console.log(isCallable(new NameBuilder('John', 'Smith')));

http://jsfiddle.net/Xotic750/58hjhtsk/1/

Anyway, I did not expect isCallable to throw an error but return false. Is this fixable?

[ljharb]

Interesting - this is caused specifically because the value isn't a function, but it has a function in its [[Prototype]]. This is definitely fixable, I'm not sure how just yet.

[ljharb]

(aside: how do you even find these? why would anyone set a function as a .prototype on a constructor?? :-p)

[Xotic750]

This test originates from commonjs-assert, the browser version of nodes assert, that is used by browserify. As I was working on fixing some issues that I was having with deepEqual and assert (for my own modules as not every developer is as welcoming/diligent as you), and so these original tests were included to make sure nothing else was broken. It's not always just me making up weird tests. :P

And as an amateur, I'm a bit like a child as I play with things (not necessarily correctly) until I break them. Javascript is a kind of hobby for me since I suffered a stroke and nobody wants to employ me any more. So I have to do something with my time so as not to go crazy, and you never know one day someone may employ me to do some Javascript from home (and I can actually earn some money again and feel a bit better about the cards that life deals one).