function disp_prompt(){
var name=prompt("请输入您的姓名","123");
if (name!=null && name!="")
{
document.write("您好," + name + "!");
}
}
<html>
<head>
<title>提示框</title>
</head>
<body>
<form action="" method="post">
<input type="button" onclick="disp_prompt()" value="显示确认框">
</form>
</body>
</html>