JavaScript：元素拖拽

/* 拖動元素函數 */
function Drag(element) {         
    element.onmousedown = function(event) {
        var distanceX=event.clientX-this.offsetLeft;
        var distanceY=event.clientY-this.offsetTop;
        if(element.setCapture)element.setCapture();
        document.onmousemove=function(event){
            var distanceLeft=event.clientX-distanceX;
            var distanceTop=event.clientY-distanceY;
            if(distanceLeft<0){
                distanceLeft=0;
            }else if (distanceLeft>document.documentElement.offsetWidth-element.offsetWidth){
                distanceLeft=document.documentElement.offsetWidth-element.offsetWidth;
            }
            if(distanceTop<0){
                distanceTop=0;
            }else if(distanceTop>document.documentElement.offsetHeight-element.offsetHeight){
                distanceTop=document.documentElement.offsetHeight-element.offsetHeight;
            }
            element.style.left=distanceLeft+'px';
            element.style.top=distanceTop+'px';
        }
        document.onmouseup=function(){
            document.onmousemove=document.onmouseup=null;
            if(element.releaseCapture)element.releaseCapture();
        }
        return false;
    }
}