// Created on savesnippets.com · https://savesnippets.com/AcYaZ2Ri4EcVMH
export function uniqueBy<T, K>(items: readonly T[], key: (item: T) => K): T[] {
  const seen = new Map<K, true>();
  const out: T[] = [];
  for (const item of items) {
    const k = key(item);
    if (!seen.has(k)) {
      seen.set(k, true);
      out.push(item);
    }
  }
  return out;
}

const users = [
  { id: 1, name: 'Alice' },
  { id: 2, name: 'Bob' },
  { id: 1, name: 'Alice dup' },
];
uniqueBy(users, u => u.id);   // [{id:1,name:'Alice'}, {id:2,name:'Bob'}]