function chkIn()
    {
  div_1.innerText = "現在你把鼠標移入層了！";
  div_1.style.font = "normal black";
 }
   function chkOut()
    {
  div_1.innerText = "現在你把鼠標移出層了！";
  div_1.style.font = "bold red";
 }

<div id="div_1" style="background-color:lightblue; width:400px; height:300px; "
       onMouseOver="chkIn()" onMouseOut="chkOut()">This is a DIV
  </div>

function   checkIn(e){
var   x=window.event.clientX;
var   y=window.event.clientY;
var   str= ' ';
for(i=0;i <document.body.children.length;i++){
            var   obj=document.body.children[i];
          if(x> obj.offsetLeft
                                  &&x <(obj.offsetLeft+obj.clientWidth)
                          &&y> obj.offsetTop
                        &&y <(obj.offsetTop+obj.clientHeight)){
                str+= ' <鼠標位于層 '+obj.id+ '范圍之內> /n ';
          }else{
                str+= ' <鼠標位于層 '+obj.id+ '范圍之外> /n ';
        }
  }
alert(str);
}
document.onclick=checkIn

if(myDiv.contains(window.event.srcElement))