ctrlKey 事件属性可返回一个布尔值，指示当事件发生时，Ctrl 键是否被按下并保持住。
event.ctrlKey=true|false|1|0
下面的例子可提示当鼠标按键被点击时 "CTRL" 键是否被按住：
运行一下 »<html>
<head>
<script>
function isKeyPressed(event)
{
if (event.ctrlKey==1)
{
alert("The CTRL key was pressed!")
}
else
{
alert("The CTRL key was NOT pressed!")
}
}
</script>
</head>
<body onmousedown="isKeyPressed(event)">
<p>Click somewhere in the document.
An alert box will tell you if you
pressed the CTRL key or not.</p>
</body>
</html>