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std/collections/group_by.ts

Deno standard library
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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;}