toMapByKey

Builds a Map from an iterable of items, keyed by a derived key. When two items derive the same key, the later item wins (last-write-wins, matching Map.prototype.set semantics).

Named toMapByKey, not keyBy (lodash’s name for the same idea) — lodash’s _.keyBy returns a plain object, not a Map; the to<Type> prefix (matching toSorted/toReversed) makes the actual return type unambiguous.

Available since v3.0.3

Import

import { toMapByKey } from '@helpers4/map';
// or, from the all-in-one package (same code, one install):
import { toMapByKey } from 'helpers4/map';

Signature

toMapByKey<T, K>(items: Iterable<T>, fn: function): Map<K, T>

Parameters

Parameter	Type	Description
items	Iterable<T>	The items to index
fn	function	Function deriving the key for each item

Returns

Map<K, T> — A Map from derived key to item

Examples

Index an array of records by id

Builds a Map keyed by a derived value, for O(1) lookup by that key.

toMapByKey([{ id: 'a', n: 1 }, { id: 'b', n: 2 }], item => item.id)
// => Map(2) { 'a' => {...}, 'b' => {...} }

Last item wins on collision

When two items derive the same key, the later one overwrites the earlier one.

toMapByKey([{ id: 'a', n: 1 }, { id: 'a', n: 2 }], item => item.id).get('a')
// => { id: 'a', n: 2 }

Name conflict

A helper named toMapByKey also exists in @helpers4/set. If you need both in the same file, rename at import with as:

import { toMapByKey as toMapByKey4map } from '@helpers4/map';
import { toMapByKey as toMapByKey4set } from '@helpers4/set';

See Name Conflicts for the full resolution guide.

Source

View source on GitHub