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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);
});
}