how to calculate value of two textfield and display its sum in third textfield autmotatically

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hi,friends i want to show the trick of how to calculate the value of two text-field and got its sum in third text-field automatically and show sum when we type some value in second text-field.

JAVASCRIPT

<script>	
function sum() {
      var txtFirstNumberValue = document.getElementById('txt1').value;
      var txtSecondNumberValue = document.getElementById('txt2').value;
      var result = parseInt(txtFirstNumberValue) + parseInt(txtSecondNumberValue);
	
      if (!isNaN(result)) {
         document.getElementById('txt3').value = result;
      }
}
</script>

HTML

<input type="text" id="txt1"  onkeyup="sum();" />
<input type="text" id="txt2"  onkeyup="sum();" />
<input type="text" id="txt3" />

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