differenceBy (Functional Programming)

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

• secondArray (readonly U[]): The array containing values whose mapped keys should be excluded.
• mapper ((item: T | U) => unknown): The function that returns the comparison key.

Returns

((array: readonly T[]) => T[]): A function that maps a readonly T[] to values whose keys are not found in secondArray.