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