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group_by.ts
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group_by.ts
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// Copyright 2018-2022 the Deno authors. All rights reserved. MIT license.
// This module is browser compatible.
/**
* Applies the given selector to each element in the given array, returning a Record containing the results as keys
* and all values that produced that key as values.
*
* Example:
*
* ```ts
* import { groupBy } from "https://deno.land/std@$STD_VERSION/collections/mod.ts";
* import { assertEquals } from "https://deno.land/std@$STD_VERSION/testing/asserts.ts";
*
* type Person = {
* name: string;
* };
*
* const people: Person[] = [
* { name: 'Anna' },
* { name: 'Arnold' },
* { name: 'Kim' },
* ];
* const peopleByFirstLetter = groupBy(people, it => it.name.charAt(0))
*
* assertEquals(peopleByFirstLetter, {
* 'A': [ { name: 'Anna' }, { name: 'Arnold' } ],
* 'K': [ { name: 'Kim' } ],
* })
* ```
*/
export function groupBy<T, K extends string>(
array: readonly T[],
selector: (el: T) => K,
): Partial<Record<K, T[]>> {
const ret: Partial<Record<K, T[]>> = {};
for (const element of array) {
const key = selector(element);
const arr = ret[key] ??= [] as T[];
arr.push(element);
}
return ret;
}