How well would this work?

[???]

I made a simple function:

const constructorof = function (data) {
	return data.constructor.toString ().split ("(") [0].split (" ") [1];
}

How well would it work?
Also, it works for custom prototypes if the custom constructor was made with a

function thing () {}

but

var thing = function () {}

doesn't work, is there any way I can make it?

[Trinithis]

I believe const is not supported by IE. Use var instead.

The constructor of

var thing = function () {}

is Function

[Trinithis]

I also doubt that there is a real use for it anyway. If you hadn't known, you can store the constructor you get through the constructor property into a variable and use it to create objects.

var s = "foo";
var c = s.constructor;
alert(c(false)); //alerts "false"
alert(typeof c(false)); //alerts "string"

[???]

It was just a random 5 second idea, but constructor is a function, and it simply getting a string for you.

[Twey]

How well would it work?

Nasty hack, but possibly necessary. The best way to do it would be:

function constructorof(o) {
  return o.constructor.name;
}

... however, some browsers don't support this (IE, I think, included), so your idea's as good as any. It won't, however, work with anonymous functions, since they don't have a name to return (by definition).