好好看手册！！！write Method Writes one or more HTML expressions to a document in the specified window. Syntax document.write(sText)Res You should not use the write or writeln methods on the current document after the document has finished loading unless you first call the open method, which clears the current document's window and erases all variables.
[个人意见,仅供参考] 单击按钮去调用aa()函数的时候,document对象已经完全载入并解析完毕了,
这是再使用document.write就相当于修改document对象的内容(也就是整个页面的内容)
所以,整个页面的内容就被清除了
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Writes one or more HTML expressions to a document in the specified window. Syntax
document.write(sText)Res
You should not use the write or writeln methods on the current document after the document has finished loading unless you first call the open method, which clears the current document's window and erases all variables.