Javascript callbacks - why are these processed in different order?

Question

How come in this example, the 'second' function is exercised second:

<script>
function first (callback){
    alert ("I am first");
    callback();
}

function second (){
    alert ("I am second");
}

first(second);
</script>

But in this example, the second function is processed first. Why does adding the () after calling the second make a difference....

<script>
function first (callback){
    alert ("I am first");
    callback();
}

function second (){
    alert ("I am second");
}

first(second());
</script>

Answer 1

Putting () after a variable holding a function will call that function.

first(second); calls first and passes the second function as an argument. first will then call callback which is the same as second

first(second()); calls second, then calls first and passes the return value of second() as the argument. first will then call undefined (and error) because that is the return value of second().