main.js

function calc(){
    var a= parseInt(document.querySelector("#Value1").value);
    var b= parseInt(document.querySelector("#Value2").value);
    var b= document.querySelector("#operator").value;
    var calculate;

    if(op == "add") {
       calculate = a + b;
    } else if (op == "min") {
        calculate= a - b;
    }    else if (op == "div") {
            calculate= a / b;
    } else if (op == "mul") {
        calculate= a * b;
    }

    console.log(calculate);
}