Lerp it! This is what drives most animations and graphics, linear interpolation.   function lerp(start, end, progress) { return start + (end - start) * progress; }   All it does is find a value in a range based on a percent. If you're moving from 0 to 300 and you're 50% the way through, how far along are you?   var x = 300 * 0.5; // 150   What if you started at 50 instead of 0? var x = 50 + (300 - 50) * 0.5; // 175   You've probably written a lerp