170 Two Sum III - Data structure design

Design and implement a TwoSum class. It should support the following operations: add and find.

add - Add the number to an internal data structure.
find - Find if there exists any pair of numbers which sum is equal to the value.

For example,

add(1); add(3); add(5);
find(4) -> true
find(7) -> false

function TwoSum() {
    this.arr = [];
}
TwoSum.prototype.add = function(k) {
    typeof this.arr[k] == 'undefined' ? this.arr[k] = 1 : this.arr[k] += 1;
}
TwoSum.prototype.find = function(val) {
    for (var i in this.arr) {
        if ((this.arr).hasOwnProperty(i) && (this.arr).hasOwnProperty(val - i)) {
            if (val == 2 * i) return this.arr[i] >= 2;
            else return true;
        }
    }
    return false;
}