asp执行js函数的问题?

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>title</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<meta name="viewport" content="width=device-width,initial-scale=1.0,maximum-scale=1.0,user-scalable=no"/>
<meta name="HandheldFriendly" content="true"/>
<meta name="MobileOptimized" content="320"/>
<script type="text/javascript" src="/js/custom.js"></script>
<%

response.write "<script>"
response.write "window.onload=function(){"
response.write "alert();"
response.write "}"
response.write "</script>"

%>

<a href="/">返回</a><br><br><br><br><br>
<a onclick="deviceVibrate()">震动</a>

<body>
</body>
</html>


js:
function deviceVibrate() {
plus.device.vibrate();
}


问题来了:我引进入页面是 执行alert()是成功的,我把alert()换成 deviceVibrate()没有效果,但我点击 震动A标签,是有效果的,何解?