function solution(input) {
    let map = {};
    let small = Number.MAX_SAFE_INTEGER, big = Number.MIN_SAFE_INTEGER;
    input.forEach(element => {
        map[element] = true;
        if (element < small) {
            small = element;
        }
        if (element > big) {
            big = element;
        }
    });

    for (let i = small; i < big; i++) {
        if (!map[i]) {
            return i;
        }
    }

}

const input = [1, 3, 6, 4, 1, 2];
console.log(solution(input));