js拖拽分析

   div.onmousedown=function(event){
         var event=event||window.event
         var diffX=event.clientX-div.offsetLeft
         var diffY=event.clientY-div.offsetTop
   }

document.onmousemove=function(event){
         var event=event||window.event
         var l=event.clientX-diffX
         var t=event.clientY-diffY
         if(t<0){
              t=0
      }
         if(t>document.documentElement.clientHeight-div.offsetHeight){
              t=document.documentElement.clientHeight-div.offsetHeight
      }
         if(l>document.documentElement.clientWidth-div.offsetWidth){
              l=document.documentElement.clientWidth-div.offsetWidth
      }
          if(l<0){
              l=0
      }
          div.style.left=l+‘px‘
          div.style.top=t+‘px
}

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