In the HTML5 spec [1] it says that if the argument is not found the method does nothing. So please remove this useless `if` :)

[1] http://dev.w3.org/html5/spec-LC/timers.html "The clearInterval() method must clear the entry identified as handle from the list of active intervals of the WindowTimers object on which the method was invoked, where handle is the argument passed to the method, if any. (If handle does not identify an entry in the list of active intervals of the WindowTimers object on which the method was invoked, the method does nothing.)"