function drag(t,p){  var point = p || null,    target = t || null,    resultX = 0,    resultY = 0;  (!point)? point = target : ''; //如果沒有拖動點，則拖動點默認為整個別拖動元素  function getPos(t){    var offsetLeft = 0,      offsetTop = 0,      offsetParent = t;    while(offsetParent){      offsetLeft+=offsetParent.offsetLeft;      offsetTop+=offsetParent.offsetTop;      offsetParent = offsetParent.offsetParent;    }    return {'top':offsetTop,'left':offsetLeft};  }  function core(){    var width = document.body.clientWidth || document.documentElement.clientWidth,      height = document.body.clientHeight || document.documentElement.clientHeight;       maxWidth = width - target.offsetWidth,      maxHeight = height - target.offsetHeight;    (resultX >= maxWidth)? target.style.left = maxWidth+'px' : (resultX > 0)?target.style.left = resultX +'px': ''; //重置默認位置。    (resultY >= maxHeight)?  target.style.top = maxHeight +'px' : (resultY > 0)?target.style.top = resultY +'px':''; //重置默認位置。    point.onmousedown=function(e){        var e = e || window.event,        coordX = e.clientX,        coordY = e.clientY,        posX = getPos(target).left,        posY = getPos(target).top;      point.setCapture && point.setCapture();  //將Mouse事件鎖定到指定元素上。      document.onmousemove=function(e){        var ev = e || window.event,          moveX = ev.clientX,          moveY = ev.clientY;        resultX = moveX - (coordX - posX); //結果值是坐標點減去被拖動元素距離瀏覽器左側的邊距        resultY = moveY - (coordY - posY);        (resultX > 0 )?((resultX < maxWidth)?target.style.left = resultX+'px' : target.style.left = maxWidth+'px') : target.style.left = '0px';         (resultY > 0 )?((resultY < maxHeight)?target.style.top = resultY+'px' : target.style.top = maxHeight+'px') : target.style.top = '0px';         ev.stopPropagation && ev.stopPropagation();         ev.preventDefault;        ev.returnValue = false;        ev.cancelBubble = true;      };    };    document.onmouseup=function(){  // 解決拖動時，當鼠標指向的DOM對象非拖動點元素時，無法觸發拖動點的onmousedown的BUG。      document.onmousemove = null;        point.releaseCapture && point.releaseCapture();  // 將Mouse事件從指定元素上移除。    };    point.onmouseup=function(e){      var e = e || window.event;      document.onmousemove = null;      point.releaseCapture && point.releaseCapture();    };  }  core();  window.onresize = core;  }

 drag(t,p) /*  * 說明  * t 表示被拖動的元素 * p 表示拖動點 */  // 注意：如果省略拖動點，默認可拖動的區域是整個被拖動元素