// @flow strict import { SYMBOL_ASYNC_ITERATOR } from '../polyfills/symbols'; import type { PromiseOrValue } from '../jsutils/PromiseOrValue'; /** * Given an AsyncIterable and a callback function, return an AsyncIterator * which produces values mapped via calling the callback function. */ export default function mapAsyncIterator<T, U>( iterable: AsyncIterable<T> | AsyncGenerator<T, void, void>, callback: (T) => PromiseOrValue<U>, rejectCallback?: (any) => PromiseOrValue<U>, ): AsyncGenerator<U, void, void> { // $FlowFixMe[prop-missing] const iteratorMethod = iterable[SYMBOL_ASYNC_ITERATOR]; const iterator: any = iteratorMethod.call(iterable); let $return: any; let abruptClose; if (typeof iterator.return === 'function') { $return = iterator.return; abruptClose = (error: mixed) => { const rethrow = () => Promise.reject(error); return $return.call(iterator).then(rethrow, rethrow); }; } function mapResult(result: IteratorResult<T, void>) { return result.done ? result : asyncMapValue(result.value, callback).then(iteratorResult, abruptClose); } let mapReject; if (rejectCallback) { // Capture rejectCallback to ensure it cannot be null. const reject = rejectCallback; mapReject = (error: mixed) => asyncMapValue(error, reject).then(iteratorResult, abruptClose); } /* TODO: Flow doesn't support symbols as keys: https://github.com/facebook/flow/issues/3258 */ return ({ next(): Promise<IteratorResult<U, void>> { return iterator.next().then(mapResult, mapReject); }, return() { return $return ? $return.call(iterator).then(mapResult, mapReject) : Promise.resolve({ value: undefined, done: true }); }, throw(error?: mixed): Promise<IteratorResult<U, void>> { if (typeof iterator.throw === 'function') { return iterator.throw(error).then(mapResult, mapReject); } return Promise.reject(error).catch(abruptClose); }, [SYMBOL_ASYNC_ITERATOR]() { return this; }, }: $FlowFixMe); } function asyncMapValue<T, U>( value: T, callback: (T) => PromiseOrValue<U>, ): Promise<U> { return new Promise((resolve) => resolve(callback(value))); } function iteratorResult<T>(value: T): IteratorResult<T, void> { return { value, done: false }; }