codewars.js

function idBestUsers(...months) {
    
    // Get user Ids present in all months
    const firstMonth = months[0];
    const filteredUsers = firstMonth.filter(userId => {
        return months.every(month => month.includes(userId));
    });

    const userCount = {};  // Stores userId instance counts as key value pair
     
    // Loop through all months and save userId count instances
    for (let i = 0; i < months.length; i++) {
      for (let j = 0; j < months[i].length; j++) {
        const user = months[i][j];
        if (filteredUsers.includes(user)) {
          userCount[user] = (userCount[user] || 0) + 1;
        }
      }
    }
    
    const values = [];  // Stores all counts for best users
    
    for (let user in userCount) {
      values.push(userCount[user]);
    }
    
    // Get unique userId count instances
    const uniqueCounts = [...new Set(values)].sort((a, b) => b - a);
    
    // Generate the desired output with array of count and user Ids
    const result = uniqueCounts.map(count => {
      const userIds = []; // Stores user Ids for same count
      for (let user in userCount) {
        if (userCount[user] === count) {
          userIds.push(user);
        }
      }
      userIds.sort();
      return [count, userIds];
    })
    
    return result;
}