Initial computed$ implementation

computed$.ts

import { Signal, isSignal } from "@angular/core";
import { toSignal, toObservable } from "@angular/core/rxjs-interop";
import { from, isObservable, Observable, ObservableInput, OperatorFunction } from "rxjs";

export function computed$<TValue, TReturn = TValue>(
  signal: Signal<TValue>,
  operator: OperatorFunction<TValue, TReturn>
): Signal<TReturn>;
export function computed$<TValue, TReturn = TValue>(
  promise: Promise<TValue>,
  initialValue: TValue,
  operator?: OperatorFunction<TValue, TReturn>
): Signal<TReturn>;
export function computed$<TValue, TReturn = TValue>(
  observable: Observable<TValue>,
  initialValue?: TValue,
  operator?: OperatorFunction<TValue, TReturn>
): Signal<TReturn>;
export function computed$<TValue, TReturn = TValue>(
  source: ObservableInput<TValue> | Signal<TValue>,
  initialValueOrOperator?: TValue | OperatorFunction<TValue, TReturn>,
  operator?: OperatorFunction<TValue, TReturn>
): Signal<TReturn> {
  const [$, op, initialValue] = toPipeableArgs(
    source,
    initialValueOrOperator,
    operator
  );

  if (!op) {
    return toSignal($, { initialValue }) as Signal<TReturn>;
  }

  return toSignal($.pipe(op), {
    initialValue: initialValue as TReturn,
  }) as Signal<TReturn>;
}

function toPipeableArgs<TValue, TReturn = TValue>(
  source: ObservableInput<TValue> | Signal<TValue>,
  initialValueOrOperator?: TValue | OperatorFunction<TValue, TReturn>,
  operator?: OperatorFunction<TValue, TReturn>
): [Observable<TValue>, OperatorFunction<TValue, TReturn>?, TValue?] {
  if (typeof source === "function" && isSignal(source)) {
    return [
      toObservable(source),
      initialValueOrOperator as OperatorFunction<TValue, TReturn>,
      source() as TValue,
    ];
  }

  if (
    source instanceof Promise ||
    ("then" in source && typeof source["then"] === "function")
  ) {
    if (
      initialValueOrOperator === undefined ||
      typeof initialValueOrOperator === "function"
    )
      throw new Error(`computed$ with Promise expects an initialValue`);

    return [from(source), operator, initialValueOrOperator as TValue];
  }

  if (isObservable(source)) {
    return [source, operator, initialValueOrOperator as TValue];
  }

  return [from(source), operator, initialValueOrOperator as TValue];
}