<script>
        var num1=document.getElementById('num1')
        var operate=document.getElementById('operate')
        var btn=document.getElementById('btn')
        var num2=document.getElementById('num2')
        var result=document.getElementById('result')
        

        btn.addEventListener('click',function(){
            switch(operate.value){
                case "+":
                result.value=Number(num1.value)+Number(num2.value);
                break;
                case "-":
                result.value=Number(num1.value)-Number(num2.value);
                break;
                case "*":
                result.value=Number(num1.value)*Number(num2.value);
                break;
                case "/":
                result.value=Number(num1.value)/Number(num2.value);
                default: break
            }
        },true)
        
    </script>