Check if array has duplicate elements

TLDR

You want a function that takes an array and checks if it contains any duplicate elements:

hasDuplicates([2, 4, 4, 6]) // true
hasDuplicates([2, 4, 6]) // false

Here’s how in JavaScript:

const hasDuplicates = arr => {
  return new Set(arr).size !== arr.length
}

Explanation

This works by first converting the given array to a Set – a data structure containing only unique values. We then check whether the number of unique elements in the set is equal to the total number of elements in the array.

If they aren’t equal, then the array must have at least one duplicate because when converting it to a set, we lost some elements (the set removes any duplicate elements).