Replacing spaces with underscores in JavaScript?

I’m trying to use this code to replace spaces with _, it works for the first space in the string but all the other instances of spaces remain unchanged. Anybody know why?

function updateKey()
    var key=$("#title").val();
    key=key.replace(" ","_");


Try .replace(/ /g,"_");

Edit: or .split(' ').join('_') if you have an aversion to REs

Edit: John Resig said:

If you’re searching and replacing through a string with a static search and a static replace it’s faster to perform the action with .split(“match”).join(“replace”) – which seems counter-intuitive but it manages to work that way in most modern browsers. (There are changes going in place to grossly improve the performance of .replace(/match/g, “replace”) in the next version of Firefox – so the previous statement won’t be the case for long.)