Q:

Javascript call function

I have been testing some code lately trying to understand javascript a little bit better. Then I came across the call() function wich I can't get to understand well.
I have the following code:
function hi(){
    console.log("hi");
}

var bye = function(param, param2){
    console.log(param);
    console.log(param2);
    console.log("bye");
}

If I call bye.call(hi(), 1, 2), I get hi 1 2 undefined
And if I call bye.cal(1,2), I get 2 undefined bye undefined
for which I understand the call() function first parameter has to be a function, followed by the amount of parameter my bye function accepts. But where is the last undefined coming from?

A:

This first parameter doesn't have to be a function.  The first parameter is the object to which the "this" variable is set to in the context of the function call.
var bye = function(param, param2){
    console.log(param);
    console.log(param2);
    console.log("bye");
    console.log(this.x)
}

t = {'x': 1};

bye.call(t, 1, 2);

And the console should show: 1, 2, "bye" and 1.
The undefined is the return value of your function.
In your first call:
bye.call(hi(), 1, 2)

You're calling hi() (so it prints 'hi'), the return value is not used, and 1 and 2 are the parameters to bye.
In your second call:
bye.cal(1,2)

1 is assigned to this.  2 is param, and param2 is undefined.

