Count unique elements in array without sorting

A quick way to do this is to copy the unique elements into an Object.

var counts = {};
for (var i = 0; i < arr.length; i++) {
    counts[arr[i]] = 1 + (counts[arr[i]] || 0);
}

When this loop is complete the counts object will have the count of each distinct element of the array.

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

This can be used to count the items in any iterable (including an Array, String, TypedArray, and arguments object).