javascript - Count unique elements in array without sorting

Question

In JavaScript the following will find the number of elements in the array. Assuming there to be a minimum of one element in the array

arr = ["jam", "beef", "cream", "jam"]
arr.sort();
var count = 1;
var results = "";
for (var i = 0; i < arr.length; i++)
{
    if (arr[i] == arr[i+1])
    {
      count +=1;
    }
    else
    {
        results += arr[i] + " --> " + count + " times
" ;
        count=1;
    }
}

Is it possible to do this without using sort() or without mutating the array in any way? I would imagine that the array would have to be re-created and then sort could be done on the newly created array, but I want to know what's the best way without sorting. And yes, I'm an artist, not a programmer, your honour.

Answer 1

The fast way to do this is with a new Set() object.

Sets are awesome and we should use them more often. They are fast, and supported by Chrome, Firefox, Microsoft Edge, and node.js.
— What is faster Set or Object? by Andrei Kashcha

The items in a Set will always be unique, as it only keeps one copy of each value you put in. Here's a function that uses this property:

function countUnique(iterable) {
  return new Set(iterable).size;
}

console.log(countUnique('banana')); //=> 3
console.log(countUnique([5,6,5,6])); //=> 2
console.log(countUnique([window, document, window])); //=> 2