MissingInteger.js

function solution(A) {
    // write your code in JavaScript (Node.js 8.9.4)
    const sorted = [...new Set(A)];
    var positives = sorted.filter(p=>p>0).sort((x,y)=>x-y);
     if(positives.length==0) return 1;
    for(let cntr=0; cntr<positives.length; cntr++)
        if(cntr+1 != positives[cntr])
            return cntr+1;
    return positives[positives.length-1] + 1;
}