allUniqueBy javascript Code Snippet in 2025

Checks if all elements in an array are unique, based on the provided mapping function.

• Use Array.prototype.map() to apply fn to all elements in arr.
• 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 mapped values to the original array.

const allUniqueBy = (arr, fn) => arr.length === new Set(arr.map(fn)).size;

allUniqueBy([1.2, 2.4, 2.9], Math.round); // true
allUniqueBy([1.2, 2.3, 2.4], Math.round); // false