js 拖拽元素并重新定位

(function () {  var dragging = false;  var boxX, boxY, mouseX, mouseY, offsetX, offsetY;  var box = document.getElementById('box');  box.onmousedown = down;  document.onmousemove = move;  document.onmouseup = up;  function down(e) {    dragging = true;    boxX = box.offsetLeft;    boxY = box.offsetTop;    mouseX = parseInt(getMouseXY(e).x);    mouseY = parseInt(getMouseXY(e).y);    offsetX = mouseX - boxX;    offsetY = mouseY - boxY;  }  function move(e) {    if (dragging) {      var x = getMouseXY(e).x - offsetX;      var y = getMouseXY(e).y - offsetY;      var width = document.documentElement.clientWidth - box.offsetWidth;      var height = document.documentElement.clientHeight - box.offsetHeight;      x = Math.min(Math.max(0, x), width);      y = Math.min(Math.max(0, y), height);      box.style.left = x + 'px';      box.style.top = y + 'px';    }  }  function up(e) {    dragging = false;  }  function getMouseXY(e) {    var x = 0, y = 0;    e = e || window.event;    if (e.pageX) {      x = e.pageX;      y = e.pageY;    } else {      x = e.clientX + document.body.scrollLeft - document.body.clientLeft;      y = e.clientY + document.body.scrollTop - document.body.clientTop;    }    return {      x: x,      y: y    };  }})()