Enter has no effect on form text field

I made a small script for a formula calculation and what I have not managed to do yet, is make the Enter act as pressing the button.
Any help please?
Here is the form:

[CODE]<form id=hba1c action=”” method=”post” name=”HbA1c_form”>
Hba1c% <input type=”text” id=”userValue” size=”15″><input type=”button” value=”&#933;&#960;&#959;&#955;&#959;&#947;&#953;&#963;&#956;&#972;&#962;” onclick=”calcResult();”>
</form>

<table id=”table_hba1c” style=”display:none;”>
<tr>
<td width=”460″></td>
<td width=”200″>&#913;&#960;&#959;&#964;&#941;&#955;&#949;&#963;&#956;&#945;</td>
</tr>
<tr>
<td>&#917;&#954;&#964;&#953;&#956;&#974;&#956;&#949;&#957;&#951; &#956;&#941;&#963;&#951; &#964;&#953;&#956;&#942; &#963;&#945;&#954;&#967;&#940;&#961;&#959;&#965;</td>
<td id=”resultl”></td>
</tr>
</table>

<script type=”text/javascript”>
function calcResult() {
n=document.getElementById(‘userValue’).value
n=parseFloat(n);
var userValue=document.forms[“hba1c”][“userValue”].value;

if (userValue==null || userValue==””)
{
alert(“Παρακαλώ καραχωρήστε την τιμή της HbA1c!”);
return false;
}

resultEl=(n*33.3-86).toFixed(1);
m=’mg/dl’;
d=’Εκτιμώμενη μέση τιμή σακχάρου:&nbsp ‘;
document.getElementById(‘resultl’).innerHTML=resultEl + ” ” + m;
document.getElementById(‘table_hba1c’).style.display=”table”;
}

</script>[/CODE]

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