IE报错，ff op 不。谁能解释一下？

</p> <div id="guoTest" onclick="alert(guoTest);guoTest=document.getElementById('guoTest');alert(guoTest)">This is testing.</div> <div id="guoTest" onclick="alert(guoTest);guoTest=guoTest.id;alert(guoTest)">This is testing.</div> <p> [Ctrl+A 全部选择 提示：你可先修改部分代码，再按运行]

在 codingforums 的回复有二：

_Aerospace_Eng_:

You can’t use an ID more than once so its likely getting confused on what id to use.

Harry Armadillo:

IE treats every name and id as a global read-only variable that references the element with that name or id. You can’t have a global variable with the same name as an element’s name or id — your variable will be overwritten by the read-only element reference.

For the sake of backwards (and IE is very backwards) compatibilty, Firefox will allow to access elements using their id as a variable (but will show a warning in the javascript console). Defining your own global variables will overwrite that element-reference.

但似乎id并不是只读的。

</p> <div id="guo" onclick="alert(this.id);this.id='shuang';alert(this.id);alert(shuang.tagName);">This is testing.</div> <p> [Ctrl+A 全部选择 提示：你可先修改部分代码，再按运行]