unionBy

Combines arrays while keeping the first value for each derived key.

Code

unionBy.ts

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

export type UnionByValue<Arrays extends ReadonlyArray<readonly unknown[]>> =
  Arrays[number][number];

/** Combines arrays while keeping the first value for every derived key. */
export const unionBy = <
  const Arrays extends ReadonlyArray<readonly unknown[]>,
  Key,
>(
  arrays: Arrays,
  getKey: UnionBySelector<UnionByValue<Arrays>, Key>,
): Array<UnionByValue<Arrays>> => {
  const seen = new Set<Key>();
  const result: Array<UnionByValue<Arrays>> = [];

  for (let arrayIndex = 0; arrayIndex < arrays.length; arrayIndex += 1) {
    if (!Object.hasOwn(arrays, arrayIndex)) continue;
    const source = arrays[arrayIndex];
    if (!Array.isArray(source)) {
      throw new TypeError("arrays must contain only arrays");
    }
    const array = source as readonly UnionByValue<Arrays>[];
    for (let index = 0; index < array.length; index += 1) {
      if (!Object.hasOwn(array, index)) continue;
      const value = array[index] as UnionByValue<Arrays>;
      const key = getKey(value, index, array);
      if (!seen.has(key)) {
        seen.add(key);
        result.push(value);
      }
    }
  }
  return result;
};

Download

wget

Terminal

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

degit

Terminal

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

Examples

unionBy.example.ts

import { unionBy } from "./unionBy";

const users = unionBy(
  [[{ id: 1 }], [{ id: 1 }, { id: 2 }]],
  (user) => user.id
);

Props

• arrays (Arrays extends readonly (readonly unknown[])[]) — Arrays to combine. Empty slots in either level 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 current array retains skipped holes and the index is relative to it.
• Returns the distinct, first-seen union of the input arrays’ element types.