05. Sum - Solution using the "Revealing Module" pattern

function getModel() {
        let num1, num2, result;

        function init(selector1, selector2, resultSelector) {
            num1 = $(selector1);
            num2 = $(selector2);
            result = $(resultSelector);
        }

        function add() {
            action((a, b) => a + b);
        }

        function subtract() {
            action((a, b) => a - b);
        }

        function action(operation) {
            let val1 = Number(num1.val());
            let val2 = Number(num2.val());
            result.val(operation(val1, val2));
        }

        let model = {init, add, subtract};
        return model;
    }