Yes JS is the Way to go
here is a small snippet that should get you going.
Carefull, this is far from professional quality code just a basic implementation using jquery.
Code:
<script src="//ajax.googleapis.com/ajax/libs/jquery/3.1.0/jquery.min.js"></script>
<input type="text" name="value1" id="textone">
<input type="text" name="value2" id="texttwo">
<input type="text" name="result" id="result">
<script>
$('#texttwo').keyup(function(){
var textone;
var texttwo;
textone = parseFloat($('#textone').val());
texttwo = parseFloat($('#texttwo').val());
var result = textone + texttwo;
$('#result').val(result.toFixed(2));
});
</script>
here is a pure javascript example (no jquery needed).
Code:
<input type="text" id="textone" onkeyup="sum();" />
<input type="text" id="texttwo" onkeyup="sum();" />
<input type="text" id="textthree" />
<script>
function sum() {
var txtFirstNumberValue = document.getElementById('textone').value;
var txtSecondNumberValue = document.getElementById('texttwo').value;
var result = parseInt(txtFirstNumberValue) + parseInt(txtSecondNumberValue);
if (!isNaN(result)) {
document.getElementById('textthree').value = result;
}
}
</script>
