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.