Getting Different Behaviors For Clicking On An Image Vs Holding The Mouse Down

I'm working on a small javascript application where I'd like to get one
behavior when a user clicks on an image (image swap), but when they
simply hold down the mouse button for a second, they get another
behavior (drop down menu).

The former seems pretty easy -- just assign the handler for the onClick
event to the swapping function. The latter, however, seems more
involved.... I know I'd assign the onMouseDown handler to a different
function, and then.... after a 1 second or so I'd want to test whether
or not the mouse buttom was still being held down. Except that I have no
idea how to do that (both the waiting part and the actual test itself).

The idea that does occur to me is to have some kind of data structure
associated with the clickable image. I could have functions associated
with the onMouseUp and onMouseDown handlers to set a flag.... as soon as
onMouseDown is called, it sets the flag to 0, then it waits 1 second,
and checks to see if the flag is 0. If onMouseUp has been called, it
will have set the flag to 1, and we call the plain old click handler. If
the flag is still 0, then I know to do the popup/drowpdown menu.

I'm a little worried about the timing between the different flows of
control, though, and I'm also wondering if there's not simply a better
way to do it.