表單驗證非空，包括空格驗證，貼代碼：
<html>
<head>
<title>input</title>
<script type="text/javascript" src="jquery.min.js"></script>
<script type="text/javascript">
$(document).ready(function(){
$('#error').hide();
$('#submit').click(function(event){
var data = $('#userId').val();
data = $.trim(data);
if(data.length < 1) {
$('#error').show();
event.preventDefault();
} else {
$('#error').hide();
}
});
});
</script>
</head>
<body>
<form id="signup" method="post" action="">
user id:<input type="text" id="userId" name="userId" /><span id="error">this field can't be blank</span><br/>
<input type="submit" id="submit" value="Submit">
</form>
</body>
</htmL>
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