adding an uncertain number of fields using javascript
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        Published on 2010-03-31T23:58:00Z
        Indexed on 
            2010/04/01
            0:03 UTC
        
        
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I'm new to javascript and a novice programmer, so this might be a really easy question to answer. I would like to loop over the values of x number of fields and add them up to display the sum in a final field. I can perform this function if I explicitly call each field, but I want to abstract this so I can deal with a flexible number of fields. Here's example code I've come up with (that's not working for me). Where am I going wrong?
<html>
<head>
<script type="text/javascript">
function startAdd(){
    interval = setInterval("addfields()",1);
}
function addfields(){
    a = document.addition.total.value;
    b = getElementByTagName("sum").value;
    for (i=0; i<=b.length; i++){
        a+=i;
    }
    return a;
}
function stopAdd(){
    clearInterval(interval);
}
</script>
</head>
<body>
<form name="addition">
 <input type="text" name="sum" onFocus="startAdd();" onBlur="stopAdd;">
+ <input type="text" name="sum" onFocus="startAdd();" onBlur="stopAdd;">
= <input type="text" name ="total">
</form>
</body>
</html>
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