Function select

libs/state/selections/src/lib/operators/select.ts:205–244

(
  ...opOrMapFn:
    | OperatorFunction<T, unknown>[]
    | string[]
    | [k: string, fn: (val: unknown) => unknown]
    | [
        keys: string[],
        fn?: (slice: unknown) => unknown,
        keyCompareMap?: KeyCompareMap<T>,
      ]
)

 * @internal
 */
export function select<T extends Record<string, unknown>>(
  ...opOrMapFn:
    | OperatorFunction<T, unknown>[]
    | string[]
    | [k: string, fn: (val: unknown) => unknown]
    | [
        keys: string[],
        fn?: (slice: unknown) => unknown,
        keyCompareMap?: KeyCompareMap<T>,
      ]
): OperatorFunction<T, unknown> {
  return (state$: Observable<T>) => {
    if (!opOrMapFn || opOrMapFn.length === 0) {
      return state$.pipe(stateful());
    } else if (isStringAndFunctionTupleGuard(opOrMapFn)) {
      return state$.pipe(stateful(map((s) => opOrMapFn[1](s[opOrMapFn[0]]))));
    } else if (isStringArrayFunctionAndOptionalObjectTupleGuard(opOrMapFn)) {
      const selectedState$: Observable<PickSlice<T & object, keyof T>> =
        state$.pipe(
          selectSlice<T & object, keyof T>(
            opOrMapFn[0] as (keyof T)[],
            opOrMapFn[2] as KeyCompareMap<{ [P in keyof T]: (T & object)[P] }>,
          ),
        );
      return typeof opOrMapFn[1] === 'undefined'
        ? selectedState$
        : selectedState$.pipe(stateful(map(opOrMapFn[1])));
    } else if (isStringArrayGuard(opOrMapFn)) {
      return state$.pipe(
        stateful(
          map((state) => opOrMapFn.reduce((acc, key) => acc?.[key], state)),
        ),
      );
    } else if (isOperateFnArrayGuard(opOrMapFn)) {
      return state$.pipe(stateful(pipeFromArray(opOrMapFn)));
    } else {
      throw new Error('wrong params passed to select');
    }
  };
}