TypeScript Array<TItem> => { [key: TKey] }: TItem

index.ts

import { toKeyedObjectWithNumber } from "./toKeyedObjectWithNumber";

var items = [
  { id: 1 },
  { id: 2 },
  { id: 3 }
];

var itemsById = toKeyedObject(items, item => item.id);

console.log(itemsById[2]);

toKeyedObject.ts

export function toKeyedObject<TItem>(
    items: Array<TItem>,
    keySelector: (item: TItem) => string) {
        return items.reduce(
            (items: { [id: string]: TItem }, item: TItem) => {
                items[keySelector(item)] = item;
                return items;
            },
            {});
    }

toKeyedObjectWithNumber.ts

export function toKeyedObjectWithNumber<TItem>(
    items: Array<TItem>,
    keySelector: (item: TItem) => number) {
        return items.reduce(
            (items: { [id: number]: TItem }, item: TItem) => {
                items[keySelector(item)] = item;
                return items;
            },
            {});
    }