Check if array has duplicates

Checks if there are duplicate values in a flat array. Use Set to get the unique values in the array. Use Set.prototype.size and Array.prototype.length to check if the count of the unique values is the same as elements in the original array.

const hasDuplicates = arr => new Set(arr).size !== arr.length;

// EXAMPLES
hasDuplicates([0, 1, 1, 2]); // true
hasDuplicates([0, 1, 2, 3]); // false

Source

https://www.30secondsofcode.org/js/s/has-duplicates