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Why do I have to omit parentheses when passing a function as an argument?

I am trying to wrap my head around as to why the following code results in a stack overflow when the parentheses are included, but do not when they omitted.

I am calling the function itself as an argument to setTimeout and it works without parantheses, but of course fails when I add them. It was my intuition to add the () after the function. Just hope somebody can clear this up for me. When are parans optional and not?

CASE 1:

var a = 1;
function foo() {
 a++;
 document.write(a);
 setTimeout(foo(), 2000)
}​ 
// RangeError: Maximum call stack size exceeded

CASE 2:

var a = 1;
function foo() {
 a++;
 document.write(a);
 setTimeout(foo, 2000)
}​
// parens are omitted on foo function and it works.

Answer*

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  • As an example, try running var foo = (function() { alert("Hello"); }) in your console; then, try running var foo = (function() { alert("Hello"); })() -- notice how the parens at the end of the statement actually invoke the function as soon as it's defined! Commented Jul 9, 2012 at 21:26
  • this makes perfect sense to me now. I was interpreting the two arguments to setTimeout(x,y) as x = what do you want to do and y = how often do you want to do it. To my misunderstanding i wanted to call and execute foo, and do it every 2 seconds. thanks for your answer! Commented Jul 9, 2012 at 21:46

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