Why does toString() work in two different ways?

Question

I've regularly used toString() for two different ways, but only recently I realized I don't understand the mechanics of what is going on.

For example I use this function to return the type of an object:

var getType = function (obj) {
    return Object.prototype.toString.call(obj).slice(8, -1);
};

getType([1,2,3]) // returns "Array"

But if I do

[1,2,3].toString()

I will get

"1,2,3"

I thought that call simply calls the function with a given this which is equal to [1,2,3].

Similarly I thought that doing [1,2,3].toString() calls toString with [1,2,3] as the this value as well.

In both cases there are no parameters and the this value is the same so what is different?

Answer 1

That's because

Array.prototype.toString !== Object.prototype.toString

For example:

Array.prototype.toString.call([1,2,3]);  // "1,2,3"
Object.prototype.toString.call([1,2,3]); // "[object Array]"