Sorting an array by occurrence of its elements efficiently

Question

I have this kind of array:

let mixedArr = ["Ship", "Ship", "Boat", "Ship", "Boat", "yacht" ]

I need a result that sorts the array by appearance, like this:

let sortedMixedArr = ["Ship", "Boat", "Yacht"]

Who can come up with a solution?

Answer 1

You need to count items and then sort entries by count

let mixedArr = ["Ship", "Ship", "Boat", "Ship", "Boat", "yacht" ]

console.log(
  sortByCount(mixedArr)
)

function sortByCount(arr) {
  return [...arr.reduce((map, item) => {
    if(map.has(item)) {
      map.set(item, map.get(item) + 1) 
    } else {
      map.set(item, 1)
    }
    
    return map
  }, new Map).entries()] // create [[name, count]]
  .sort(([_, a], [__, b]) => b - a) // sort by count numerically
  .map(([name]) => name) // extract names
}

Answer 2

A shorter approach by using the keys of the map and sort them by the count.

var array = ["Ship", "Ship", "Boat", "Ship", "Boat", "Yacht"],
    map = array.reduce((m, v) => m.set(v, (m.get(v) || 0) + 1), new Map),
    result = [...map.keys()].sort((a, b) => map.get(b) - map.get(a));

console.log(result);