unionWith (Functional Programming)

Creates a function that combines arrays using a custom equality function. Use it with pipe.

const result = pipe(array, unionWith(secondArray, areItemsEqual));

INFO

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

Usage

unionWith keeps the first value from the combined arrays for which areItemsEqual does not match an already kept value.

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

pipe(
  [{ id: 1 }, { id: 2 }],
  unionWith([{ id: 2 }, { id: 3 }], (a, b) => a.id === b.id)
); // => [{ id: 1 }, { id: 2 }, { id: 3 }]

Parameters

• secondArray (readonly T[]): The array to combine after the piped array.
• areItemsEqual ((item: T, other: T) => boolean): The function that decides whether two values are equal.

Returns

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