update

node_modules/rxjs/src/internal/firstValueFrom.ts

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+import { Observable } from './Observable';
+import { EmptyError } from './util/EmptyError';
+import { SafeSubscriber } from './Subscriber';
+
+export interface FirstValueFromConfig<T> {
+  defaultValue: T;
+}
+
+export function firstValueFrom<T, D>(source: Observable<T>, config: FirstValueFromConfig<D>): Promise<T | D>;
+export function firstValueFrom<T>(source: Observable<T>): Promise<T>;
+
+/**
+ * Converts an observable to a promise by subscribing to the observable,
+ * and returning a promise that will resolve as soon as the first value
+ * arrives from the observable. The subscription will then be closed.
+ *
+ * If the observable stream completes before any values were emitted, the
+ * returned promise will reject with {@link EmptyError} or will resolve
+ * with the default value if a default was specified.
+ *
+ * If the observable stream emits an error, the returned promise will reject
+ * with that error.
+ *
+ * **WARNING**: Only use this with observables you *know* will emit at least one value,
+ * *OR* complete. If the source observable does not emit one value or complete, you will
+ * end up with a promise that is hung up, and potentially all of the state of an
+ * async function hanging out in memory. To avoid this situation, look into adding
+ * something like {@link timeout}, {@link take}, {@link takeWhile}, or {@link takeUntil}
+ * amongst others.
+ *
+ * ## Example
+ *
+ * Wait for the first value from a stream and emit it from a promise in
+ * an async function
+ *
+ * ```ts
+ * import { interval, firstValueFrom } from 'rxjs';
+ *
+ * async function execute() {
+ *   const source$ = interval(2000);
+ *   const firstNumber = await firstValueFrom(source$);
+ *   console.log(`The first number is ${ firstNumber }`);
+ * }
+ *
+ * execute();
+ *
+ * // Expected output:
+ * // 'The first number is 0'
+ * ```
+ *
+ * @see {@link lastValueFrom}
+ *
+ * @param source the observable to convert to a promise
+ * @param config a configuration object to define the `defaultValue` to use if the source completes without emitting a value
+ */
+export function firstValueFrom<T, D>(source: Observable<T>, config?: FirstValueFromConfig<D>): Promise<T | D> {
+  const hasConfig = typeof config === 'object';
+  return new Promise<T | D>((resolve, reject) => {
+    const subscriber = new SafeSubscriber<T>({
+      next: (value) => {
+        resolve(value);
+        subscriber.unsubscribe();
+      },
+      error: reject,
+      complete: () => {
+        if (hasConfig) {
+          resolve(config!.defaultValue);
+        } else {
+          reject(new EmptyError());
+        }
+      },
+    });
+    source.subscribe(subscriber);
+  });
+}