The onunload event occurs when the user leaves the page .
onunload事件在用户离开页面时发生。
OnUnload & This function is called just before a page is being switched out .
onUnload&此函数只在页面被关闭前调用。
For example , the tag has the onload and onunload attributes .
例如，标记拥有onload和onunload属性。
onUnload