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In order to satisfy the O(N) time-complexity, construct a Set() in O(N) time and space complexity, then use a while loop which is considered constant time relative to N, since the specified range of numbers in A is independent of N:

function solution(A) {
  const set = new Set(A);
  let i = 1;

  while (set.has(i)) {
    i++;
  }

  return i;
}