@lou.codes/iterables/dist/groupBy.js

🔁 Iterable and AsyncIterable utils

import { EMPTY_ARRAY, EMPTY_OBJECT } from "@lou.codes/constants/empty.js";
import { set } from "@lou.codes/utils";
import { reduce } from "./reduce.js";
/**
 * Groups values of an iterable in an object based on the output of the
 * `grouper` function.
 *
 * @category Reducers
 * @example
 * ```typescript
 * const groupByType = groupBy((value: number) => number % 2 === 0 ? "even" : "odd");
 * groupByType([1, 2, 3, 4, 5]); // { even: [2, 4], odd: [1, 3, 5] }
 * ```
 * @param grouper Grouper function.
 * @returns Object with grouped values.
 */
export const groupBy = grouper =>
	reduce(item => {
		const group = grouper(item);
		return groups =>
			set(group)([...(groups[group] ?? EMPTY_ARRAY), item])(groups);
	})(EMPTY_OBJECT);