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.