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// @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 };
}
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