I need some help with a simple calculator I am doing with javascript.

The js code is as follows (my teacher want's me to use 4 functions, for each kind of operation):

<script>



            function plus(a,b) {

                return (a + b);

            }



            function minus(a,b) {

                return (a - b);

            }



            function multiply(a,b) {

                return (a * b);

            }



            function divide(a,b) {

                return (a / b);

            }



            function calc() {



                var x = document.getElementById(oper1).value;

                var y = document.getElementById(operx).value;

                var z = document.getElementById(oper2).value;

                var w = document.getElementById(resul).value;



                switch (y) {

                    case '0':

                        w = plus(x, z);

                        break;



                    case '1':

                        w = minus(x, z);

                        break;



                    case '2':

                        w = multiply(x, z);

                        break;



                    case '3':

                        w = divide(x, z);

                        break;



                    default:

                        w = Don't really know..;

                }



            }



</script>

<input type=text id=oper1 value=>



<select id=operx>

    <option value=0>SUMAR</option>

    <option value=1>RESTAR</option>

    <option value=2>MULTIPLICAR</option>

    <option value=3>DIVIDIR</option>

</select>



<input type=text id=oper2 value=>

<input type=button onClick=calc(); value==>

<input type=text id=resul value=>

My code isn't working, in fact is not responding anything and I don't see any errors so I can debug... could anyone tell me if you see my mistake here? I've tried hundreds of combinations but without having an debug console or something.