getResult.ts

export type ResolvedResult<T> = [error: null, value: T];
export type RejectedResult = [error: unknown, value: null];
export type Result<T> = ResolvedResult<T> | RejectedResult;

export function getResult<T>(promise: PromiseLike<T>): Promise<ResolvedResult<T> | RejectedResult> {
  return Promise.resolve(promise)
    .then(value => [null, value] as [null, T])
    .catch(error => [error, null] as [unknown, null]);
}

export function isResolved<T>(result: Result<T>): result is ResolvedResult<T> {
  return result[0] != null;
}
export function isRejected<T>(result: Result<T>): result is RejectedResult {
  return !isResolved(result);
}
export function assertResolved<T>(result: Result<T>, message?: string): asserts result is ResolvedResult<T> {
  if (isRejected(result)) {
    throw message ? new Error(message) : result[0];
  }
}
export function assertRejected<T>(result: Result<T>, message?: string): asserts result is RejectedResult {
  if (isResolved(result)) {
    throw new Error(message);
  }
}

async function test() {
  const result = await getResult(Promise.resolve(2));
  assertResolved(result);
  const [, data] = result;

  return 2 + data;
}