For AI agents: the complete documentation index is available at /llms.txt, the full documentation bundle is available at /llms-full.txt, and this page is available as Markdown at /reference/fp/unionWith.md.
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    unionWith (Functional Programming) es-toolkit

    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

    secondArrayreadonly T[]required

    The array to combine after the piped array.

    areItemsEqual(item: T, other: T) => booleanrequired

    The function that decides whether two values are equal.

    Returns

    A function that maps a readonly T[] to a union by custom equality.

    (array: readonly T[]) => T[]
    Source: es-toolkit

    Re-exported verbatim from es-toolkit. Implementation, edge cases, and performance behavior are owned upstream. This page mirrors the documentation at the pinned version; the linked source is authoritative.