Code:
<script>
function muloperator()
{
 var val1 = parseInt(document.getElementById("txt1").value);
        var val2 = parseInt(document.getElementById("txt2").value); 
       var ans=document.getElementById("demo")

}

  sum()
 {
      ans.value = val1+ val2;
}
minus()
{
ans.value = val1 - val2;
}
mul()
{
...