Demo test question on Codility.

smallest-in-array-positive.js

const sorted = (a) => Array.prototype.sort.call(a, (a, b) => a - b);
const tail = (n, a) => Array.prototype.slice.call(a, n);
const abs = Math.abs;

const solution = (a) => {
    const sa = sorted(a);
    let last = abs(a[0]);

    for (const x of tail(1, sa)) {
        const negative = x < 0;
        const ax = abs(x);

        if (ax === 0 || last === ax) {
            continue;
        }

        if (abs(ax - last) > 1) {
            if (negative) {
                return last - 1;
            } else {
                return last + 1;
            }
        }

        last = ax;
    }
};

console.log(solution([1, 3, 7, 4, 1, 2]) === 5);
console.log(solution([-1, -3]) === 2);