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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); }); }