const val1 = 10;
        const val2 = 5;
        let operat = "-";
        console.log(cal(val1, val2, operat));
        operat = "+";
        console.log(cal(val1, val2, operat));
        function cal(a, b, op) {
            if (op == "-") {
                return a - b;
            } else {
                return a + b;
            }
        }