codility.js

function solution(A) {
    // write your code in JavaScript (Node.js 8.9.4)
    let sortedA = A.sort();
    
    for (let i = 0; i < sortedA.length; i++) {
        console.log(sortedA.indexOf(sortedA[i] + 1))
        if (sortedA.indexOf(sortedA[i] + 1) === -1) {
            return sortedA[i] + 1;
        }
    }
}