flow like the river

VISUALIZACION/node_modules/graphql/subscription/mapAsyncIterator.js.flow

@@ -0,0 +1,74 @@
+// @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 };
+}