OnChange show

<script>
function intval(val){
    return parseInt(val) || 0;
}
function calculateSum(){
   document.getElementById("inC").value = intval(document.getElementById("inA").value) + intval(document.getElementById("inB").value);
}
</script>

A: <input type="text" name="a" id="inA" onchange="calculateSum()"><br>
B: <input type="text" name="b" id="inB" onchange="calculateSum()"><br>
C: <input type="text" name="c" id="inC" readonly><br>