allUnique

Checks if all elements in an array are unique.

• Create a new Set from the mapped values to keep only unique occurrences.
• Use Array.prototype.length and Set.prototype.size to compare the length of the unique values to the original array.

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

allUnique([1, 2, 3, 4]); // true
allUnique([1, 1, 2, 3]); // false