waitForSubscription helper function for testing graphql subscriptions

wait-for-subscription-with-trigger.ts

/**
 * Waits for a subscription after a trigger function is called. The trigger function is delayed to
 * properly time the subscription return event.
 */
const waitForSubscription = async <TResult = any>(
  subscription: DocumentNode,
  trigger: () => Promise<void>
): Promise<ExecutionResult<TResult>> => {
  const iterator = await subscribe<TResult>(
    schema,
    subscription,
    {},
    isFunction(options.context)
      ? options.context({} as ExpressContext)
      : options.context
  );

  if (!isAsyncIterable(iterator)) {
    return iterator;
  }

  const [res] = await Promise.all([
    (async () => {
      for await (const next of iterator) {
        return next;
      }

      throw new Error('iterator completed without returning a result');
    })(),
    (async () => {
      // ensure that trigger occurs after the iterator begins
      await nextTick();
      await trigger();
    })(),
  ]);

  return res;
};

wait-for-subscription.ts

export async function waitForSubscription<TResult, TContext>(
  subscription: DocumentNode,
  context: TContext
): Promise<ExecutionResult<TResult>> {
  const iterator = await subscribe<TResult>(schema, subscription, {}, context);

  if (!isAsyncIterable(iterator)) {
    return iterator;
  }

  for await (const next of iterator) {
    return next;
  }

  throw new Error('iterator completed without returning a result');
}

I updated this with the following changes from the fork:

1. It returns an ExecutionResult which has a similar shape to apollo-server-testing's results, so tests can be a bit more consistent.
2. Rearranged it so there's no need for new Promise, it's just an async function now.