javascript in aspx

hello, i did the test below in a normal aspx page but it is not working:


<form id="login">
<div style="border:solid 1px #deb887;background-color:#f5f5dc;font-size:8pt;font-weight:bold;text-align:center;width:250px;padding:10px 0px 10px 0px;">CODICE CLIENTE<br />
<input type="text" maxlength="6" id="user_id" name="user_id" />&nbsp;<input type="button" value="Accedi" onclick="login()" /></div>
</form>

<script language="javascript" type="text/javascript">
    function login(){
        var id = user_id;
        location.href ="previsioni-forex.aspx?id="+id;
    }
</script>


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