Codility MaxCounters Solution in Javascript - 66% score

MaxCounters.js

/*
You are given N counters, initially set to 0, and you have two possible operations on them:

• increase(X) − counter X is increased by 1,
• max counter − all counters are set to the maximum value of any counter.

A non-empty zero-indexed array A of M integers is given. This array represents consecutive operations:

• if A[K] = X, such that 1 ≤ X ≤ N, then operation K is increase(X),
• if A[K] = N + 1 then operation K is max counter.
*/

var max;

function solution(N, A) {
    var counters = Array.apply(null, Array(N)).map(function(){return 0;});
    var NP = N + 1;
    
    for (var K in A){
        if (A[K] >= 1 && A[K] <= N){
            counters[A[K] - 1]++;
        }else if(A[K] === NP){
            max = Math.max.apply(this, counters);
            counters = counters.map(retMax);
        }
    }
    
    return counters;
}

function retMax(){ return max; }