unionBy (Functional Programming)

Creates a function that combines arrays by a mapped key without duplicates. Use it with pipe.

const result = pipe(array, unionBy(secondArray, mapper));

::: info

Prefer the original es-toolkit unionBy in ordinary code. Use this fp variant when composing transformations with pipe.

:::

Usage

unionBy compares the values returned by mapper. It keeps the first value for each mapped key from the piped array, then from secondArray.

import { pipe, unionBy } from 'es-toolkit/fp';

pipe(
  [{ id: 1 }, { id: 2 }],
  unionBy([{ id: 2 }, { id: 3 }], item => item.id)
); // => [{ id: 1 }, { id: 2 }, { id: 3 }]

Parameters

• secondArray (readonly T[]): The array to combine after the piped array.
• mapper ((item: T) => U): The function that returns the key used for uniqueness.

Returns

((array: readonly T[]) => T[]): A function that maps a readonly T[] to a union by mapped key.