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.