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/differenceBy.md.
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    differenceBy (Functional Programming) es-toolkit

    Creates a function that excludes values by a mapped key. Use it with pipe.

    const result = pipe(array, differenceBy(secondArray, mapper));
    Info

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

    Usage

    differenceBy compares the values returned by mapper. Values from the piped array are kept when their mapped key does not appear in secondArray.

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

pipe(
  [{ id: 1 }, { id: 2 }],
  differenceBy([2], value => (typeof value === 'number' ? value : value.id))
); // => [{ id: 1 }]

    Parameters

    secondArrayreadonly U[]required

    The array containing values whose mapped keys should be excluded.

    mapper(item: T | U) => unknownrequired

    The function that returns the comparison key.

    Returns

    A function that maps a readonly T[] to values whose keys are not found in secondArray.

    (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.