I don't know why console.log(Set.current_index) shows 0 instead of 3.

var Set = (function() {
    var set = [];
    var index = 0;

    function contains(set, e) {
        for (var i = 0; i < set.length; i++) {
            if (set[i] === e) {
                return true;
            }
        }
        return false;
    }

    var add = function(e) {
        if (!contains(set, e)) {
            set[index++] = e;
        }
    }

    var show = function() {
        for (var i = 0; i < set.length; i++) {
            console.log(set[i]);
        }
    }

    return {
        add: add,
        show: show,
        current_index: index
    };
})();?

Set.add(20);
Set.add(30);
Set.add(40);
Set.show();
console.log(Set.current_index);