waitForSubscription helper function for testing graphql subscriptions

wait-for-subscription.ts

export function waitForSubscription<TResult, TContext>(schema: GraphQLSchema, subscription: DocumentNode, context: TContext) {
  return new Promise<TResult | ReadonlyArray<GraphQLError>>(async resolve => {
    const iterator = await subscribe(schema, subscription, {}, context)

    if (isAsyncIterable(iterator)) {
      for await (const next of iterator) {
        const val: ExecutionResult<TResult> = next

        if (val.errors) resolve(val.errors)

        const result = val.data
        
        if (result) resolve(result)
      }
    }

    if ('errors' in iterator) {
      resolve(iterator.errors)
    }
  })
}