简单的拖动功能

drag.js

	var box = document.querySelector('#box');
	box.onmousedown = function (e) {
		box.style.position = 'absolute';
		var diffX = e.clientX - box.offsetLeft;
		var diffY = e.clientY - box.offsetTop;

		document.onmousemove = function (e) {
			box.style.left = e.clientX - diffX + 'px';
			box.style.top = e.clientY - diffY + 'px';
		}

		document.onmouseup = function () {
			document.onmousemove = null;
			document.onmouseup = null;
		}
	}