keyBy

Indexes array values by a derived property key.

Code

keyBy.ts

export type KeyByKeySelector<Value, Key extends PropertyKey> = (
  this: void,
  value: Value,
  index: number
) => Key;

/**
 * Indexed values are optional because a selector may not produce every key in
 * its return type for a particular input.
 */
export type KeyByResult<Value, Key extends PropertyKey> = Partial<
  Record<Key, Value>
>;

/** Indexes values by a derived property key, with later values replacing earlier ones. */
export const keyBy = <Value, Key extends PropertyKey>(
  array: readonly Value[],
  getKey: KeyByKeySelector<Value, Key>
): KeyByResult<Value, Key> => {
  const result = Object.create(null) as KeyByResult<Value, Key>;
  for (let index = 0; index < array.length; index += 1) {
    if (!Object.hasOwn(array, index)) continue;
    const value = array[index] as Value;
    Reflect.set(result, getKey(value, index), value);
  }
  return result;
};

Download

wget

Terminal

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

degit

Terminal

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

Examples

keyBy.example.ts

import { keyBy } from "./keyBy";

const usersById = keyBy(
  [{ id: "ada", active: true }, { id: "lin", active: false }],
  (user) => user.id
);

const ada = usersById.ada;
// { id: "ada", active: true } | undefined

Props

• array (readonly T[]) — Values to index. Empty slots are skipped.
• getKey (KeyByKeySelector<T, Key>) — Produces each object key and is invoked without a receiver.
• Returns Partial<Record<Key, T>> with a null prototype. A value is undefined when its key was not produced.
• Later duplicate keys replace earlier values.