Count unique elements in array without sorting

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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).

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