老师解释一下这段代码的意思

相关代码：

 function handleMove(ev) {
                ev.preventDefault();

                isTouchMove = true;

                var touch = ev.changedTouches[0];
                var diffPoint = {};
                var movePoint = {
                    x: 0,
                    y: 0
                };

                diffPoint.x = touch.pageX - startPoint.x;
                diffPoint.y = touch.pageY - startPoint.y;
​问题描述：如果开始点在（200,100）位置，而移动时touch.pageX是100，那么diffPoint.x就是
-100，这样理解对吗？那么，传入move函数的x就是负值了，不就超出body 了吗？
                if (options.x) {
                    movePoint.x = diffPoint.x + curPoint.x;
                }
                if (options.y) {
                    movePoint.y = diffPoint.y + curPoint.y;
                }

                move(el, movePoint.x, movePoint.y);
            }