I had a difficult-to-spot logic error in JS.

There is a sprite class, and code to update its position and deactivate/stop
it if it is off-screen. It was working, but I had duplicate code in different
places, so I decided to consolidate it in one place. It made sense to 
move the function to a method, so that's what I did.

But then the sprites refused to move. 
I found that they would deactivate/stop every tick of the timer function.
I realized that the error stems from these lines:

    tempX = sprites[i].floatX + sprites[i].DX;
    tempY = sprites[i].floatY + sprites[i].DY;

The reason is simple and dumb: since this code is within a method,
i.e. called by sprites[i].Update(), the "sprites[i]." does not have the
same meaning inside it. The fix is easy:

    tempX = this.floatX + this.DX;
    tempY = this.floatY + this.DY;