// Snap object depending on the snapToSize
// Can be used for window dragging or in games etc.
// if the side of my square in my grid is 250, the snapToSize parameter would be 250. 
// If it is 100, the snapToSize parameter would be 100.

var snapToSize:int = 5;

snapClip_mc.x = mouseX - mouseX % snapToSize;
snapClip_mc.y = mouseY - mouseY % snapToSize;

Using modulo to snap an object

object, math

ActionScript 3