most_watched.js

const movies = require('/home/movies');
const users = require('/home/users');

function topWatchlistedMoviesAmongFriends(user_id, users, movies) {
  // get current user
  const currentUser = users.find((user) => user.userId === user_id)

  // return an empty array if a current user can't be found
  if (!currentUser) {
    return []
  }

  // access array of friends of current user
  const { friends } = currentUser

  // get a list of shared movies between all friends
  const sharedMovies = []
  movies.forEach((movie) => {
    const { watchlist } = movie
    friends.forEach((friend) => {
      if (watchlist.includes(friend)) {
        sharedMovies.push(movie)
      }
    })
  })

  // add a count property to each movie object to count how many friends have it in their list
  for (let movie of sharedMovies) {
    const count = sharedMovies.filter((m) => movie === m).length
    movie.count = count
  }

  // make a set of unique movies, remove any duplicates
  const sharedMoviesSet = new Set(sharedMovies)

  // convert movies back into an array and sort them based on count 
  const uniqueMovies = Array.from(sharedMoviesSet).sort((a, b) => b.count - a.count)

  // make a data structure of movie arrays based on count
  const movieObj = {}
  uniqueMovies.forEach((movie) => {
    if (!movieObj[movie.count]) {
      movieObj[movie.count] = []
      movieObj[movie.count].push(movie.title)
    } else {
      movieObj[movie.count].push(movie.title)
    }
  })

  // sort each movie array nested in the object and unshift it to the front of the array
  let result = []
  for (let key in movieObj) {
    movieObj[key] = movieObj[key].sort()
    result.unshift(movieObj[key])
  }

  // flatten the array to have a single array instead of an array of arrays
  const flatArray = result.flat()

  // return only 4 items if length is longer than 4
  if (flatArray.length > 4) {
    return flatArray.slice(0, 4)
  } else {
    return flatArray
  }
};