[Codility] MissingInteger

function solution(A) {
    // write your code in JavaScript (Node.js 8.9.4)
    const len = A.length
    let prev = 0
    A.sort((a,b) => a - b)
    
    if(A[len - 1] <= 0) return 1
    
    for (let i = 0; i < len;i++) {
        if(A[i] > 0) {
            if(A[i] <= prev + 1) {
                prev = A[i]
            } else {
                return prev + 1
            }
        }
    }
    return prev + 1
}