Bounce

gistfile1.js

box = {x: 10, y: 10, width: 40, height: 50, vX: 5, vY: 5};
bounds = {left: 0, right: 800, top: 0, bottom: 400};

frame() {

    box.nX = box.x + box.vX;
    box.nY = box.y + box.vY;

    if(box.nX < bounds.left) box.vX *= -1;
    if(box.nX + box.width > bounds.right) box.vX *= -1;
    if(box.nY < bounds.top) box.vY *= -1;
    if(box.nY + box.height > bounds.bottom) box.vY *= -1;

    box.x += box.vX;
    box.y += box.vY;
}