setTimeout with or without anonymous function? What's the difference?

Question

I used this code (followed by an xmlhttprequest that fills the "tcap" textarea):

st=setTimeout(checkme(),4000)

where checkme() is:

function checkme() {
    if (typeof (st) != 'undefined') clearTimeout(st)
    if (document.getElementById("tcap").innerHTML.length > 0) {
        document.getElementById('waitmsg').style.display = 'none'
    } else {
        st = setTimeout(checkme(), 1000)
    }
}  

If I run it, it freezes Firefox 19 with no error message. But if I replace the first argument (both in code and in the checkme() function) with:

st=setTimeout(function(){checkme()},4000)

it works correctly. So my question is: what's the difference in calling the checkme() function with or without the anon function? Why in the first case it freezes Firefox?

Thanks

Answer 1

You need to remove the parens in

st=setTimeout(checkme(),4000)

so instead:

st=setTimeout(checkme,4000)

otherwise, the function is invoked right away.

Since you have the same error inside the checkme function, it probably kills your browser due to unbounded recursion.

Answer 2

setTimeout accepts a function as an argument, and the correct way to pass a function as an argument is either defining it as an anonymous function, or just providing the function name. If you use parenthesis(brackets), you aren't actually passing a function: You are executing the function and passing the result of the function to setTimeout.

Hence, when specifying a function in setTimeout, and anywhere else you need to pass a function as an argument, you should not use parenthesis.

Answer 3

You shouldn't be using the parenthesis within the setTimeout function. You should only be passing in a reference to the method. What you are doing is invoking the method and passing the return value in to the set timeout method.