@monstermann/fn/dist/array/groupBy.js

A utility library for TypeScript.

import { dfdlT } from "@monstermann/dfdl";

//#region src/array/groupBy.ts
/**
* `groupBy(array, grouper, transform?)`
*
* Groups elements in `array` by the result of calling `grouper` function on each element, optionally transforming each element with `transform`, returning an object with keys as group values and values as arrays of elements.
*
* ```ts
* groupBy(
*     [1, 2, 3, 4],
*     (x) => (x % 2 === 0 ? "even" : "odd"),
*     (x) => x * 10,
* ); // { even: [20, 40], odd: [10, 30] }
* ```
*
* ```ts
* pipe(
*     [1, 2, 3, 4],
*     groupBy(
*         (x) => (x % 2 === 0 ? "even" : "odd"),
*         (x) => x * 10,
*     ),
* ); // { even: [20, 40], odd: [10, 30] }
* ```
*/
const groupBy = dfdlT((target, by, transform) => {
	return target.reduce((acc, value, idx) => {
		const key = by(value, idx, target);
		acc[key] ??= [];
		acc[key].push(transform ? transform(value, key, idx, target) : value);
		return acc;
	}, {});
}, (args) => Array.isArray(args[0]));

//#endregion
export { groupBy };