uniqueBy

Keeps the first value for each distinct derived key.

Code

uniqueBy.ts

export type UniqueBySelector<Value, Key> = (
  this: void,
  value: Value,
  index: number,
  array: readonly (Value | undefined)[],
) => Key;

/** Keeps the first value for every distinct derived key. */
export const uniqueBy = <const T, Key>(
  array: readonly T[],
  getKey: UniqueBySelector<T, Key>,
): T[] => {
  const seen = new Set<Key>();
  const result: T[] = [];
  for (let index = 0; index < array.length; index += 1) {
    if (!Object.hasOwn(array, index)) continue;
    const value = array[index] as T;
    const key = getKey(value, index, array);
    if (seen.has(key)) continue;
    seen.add(key);
    result.push(value);
  }
  return result;
};

Download

wget

Terminal

wget -O src/lib/uniqueBy.ts https://raw.githubusercontent.com/jrTilak/lazykit/HEAD/registry/functions/uniqueBy.ts

degit

Terminal

temp_dir="$(mktemp -d)" && bunx degit jrTilak/lazykit/registry/functions "$temp_dir" && mv "$temp_dir/uniqueBy.ts" src/lib/uniqueBy.ts && rm -r "$temp_dir"

Examples

uniqueBy.example.ts

import { uniqueBy } from "./uniqueBy";

const users = uniqueBy(
  [{ id: 1, name: "Ada" }, { id: 1, name: "A." }],
  (user) => user.id
);

Props

• array (readonly T[]) — Values to deduplicate. Empty slots are skipped.
• getKey ((value: T, index, array: readonly (T | undefined)[]) => Key) — Produces the identity key and is invoked without a receiver. value is always present; the original array retains skipped holes.
• Returns a new T[] in first-seen order.