html==========

 
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css================

#p1{overflow:scroll;width: 100px;height: 100px;border: 1px solid #000;}

js======================

var p=document.getElementById('p1'); alert(p.clientHeight); //86 alert(p.scrollHeight); //276 alert(document.body.clientHeight); //1390 alert(document.body.scrollHeight); //1410

为什么p.clientHeight不能取到滚动条之后的高度，document.body.clientHeight可以取到滚动条之后的高度？