Generic indexBy function + toDictionary RxJS Operator

toDictionary.ts

import { type Observable } from 'rxjs';
import { map } from "rxjs/operators";

//GENERIC toDictionary OPERATOR + indexBy FUNCTION RETURN Map<indexOf, item> TO OTTIMIZE O(1) EXIST/RETRIVE OF ITEM BY idxOf (.has(idx)/.get(idx))
export type Indexer<T extends {}> = keyof T | ((item: T) => unknown);
export type IndexOf<T, I extends Indexer<T>> = I extends keyof T ? T[I] : I extends (...args: any) => any ? ReturnType<I> : never;
export const indexBy = <T, I extends Indexer<T>>(array: T[], keyOrIndexer: I): Map<IndexOf<T, I>, T> => {
    if (!array) return new Map<IndexOf<T, I>, T>();
    const idexerFn: (item: T) => IndexOf<T, I> = (
        typeof keyOrIndexer === "function" ? keyOrIndexer : (item: T) => item[keyOrIndexer as keyof T]
    ) as any;
    return new Map(array.map((item) => [idexerFn(item), item]));
};
export const toDictionary = <T, I extends Indexer<T>>(keyOrIndexer: I) => map((array: T[]) => indexBy(array, keyOrIndexer));