Firefox setTimeout(func, ms) sending default parameters to callback

Question

I am unable to find out more about this default parameter that I ran across and was hoping that someone could point out an explanation.

In Firefox (3.6 in this case) if

Answer 1

From MDC

Gecko passes an extra parameter to the callback routine, indicating the "lateness" of the timeout in milliseconds.

Because the "actual" delay maybe longer than the delay specified in the setTimeoutcall, the "lateness" will be zero if the function was called exactly after the delay specified, non-zero otherwise.

Answer 2

You're correct that it's the offset. It'll usually be zero (meaning it was called when it should have been), but if the JS engine is backed up, it'll be higher, or it can even be negative.

reference: https://developer.mozilla.org/en/DOM/window.setTimeout

(see the part in yellow)