solution number 2

let solution = (n, users) => {
  let answer = [];
  let totalUsers = users.length;
  let countDuplicateStageUsers = users.reduce((prev, curr) => {
    prev[curr] = (prev[curr] || 0) + 1;
    return prev;
  }, {});

  // looping stages
  for (let i = 1; i <= n; i++) {
    let currObjKey = Object.keys(countDuplicateStageUsers)[i - 1];
    let failureStageCount = countDuplicateStageUsers[currObjKey] || 0;
    let failureCount = 0;

    for (var key in countDuplicateStageUsers) {
      if (countDuplicateStageUsers.hasOwnProperty(key)) {
        if (key <= currObjKey) {
          failureCount += countDuplicateStageUsers[key];
        }
      }
    }
    let totalCurrFaileur = totalUsers - failureCount;
    let restUsers = failureStageCount + totalCurrFaileur;
    let totalFailure = i === Number(currObjKey) ? failureStageCount : 0;
    let failureRate = i === restUsers ? 1 : totalFailure / restUsers;

    answer.push({
      stage: i,
      failureRate,
    });
  }

  let sortDescAnswer = answer.sort((a, b) => b.failureRate - a.failureRate);

  return sortDescAnswer.map((data) => data.stage);
};

let n = 5;
let users = [2, 1, 2, 6, 2, 4, 3, 3];
let users1 = [4, 4, 4, 4];
// [3,4,2,1,5]

console.log(solution(n, users));