createPendingValue.ts

export function createPendingValue<T, Ref = any, S = never>(ref: Ref, key: keyof Ref) {
  let resolve!: (v: T) => void;
  let reject!: (reason?: any) => void;

  const p = new Promise<T>((_resolve, _reject) => {
    resolve = value => {
      ref[key] = value as Ref[keyof Ref];
      _resolve(value);
    };
    reject = reason => {
      ref[key] = createPendingValue<T, Ref, S>(ref, key) as Ref[keyof Ref];
      _reject(reason);
    };
  });

  const assigned = Object.assign(p, { resolve, reject, isPending: true } as const);

  return p as (typeof assigned & S) | (T & { isPending?: false });
}

export type PendingValue<T, Ref, S> = ReturnType<typeof createPendingValue<T, Ref, S>>;