import { Observable } from './Observable';
|
import { EmptyError } from './util/EmptyError';
|
|
export interface LastValueFromConfig<T> {
|
defaultValue: T;
|
}
|
|
export function lastValueFrom<T, D>(source: Observable<T>, config: LastValueFromConfig<D>): Promise<T | D>;
|
export function lastValueFrom<T>(source: Observable<T>): Promise<T>;
|
|
/**
|
* Converts an observable to a promise by subscribing to the observable,
|
* waiting for it to complete, and resolving the returned promise with the
|
* last value from the observed stream.
|
*
|
* 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 complete. If the source
|
* observable does not 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 last value from a stream and emit it from a promise in
|
* an async function
|
*
|
* ```ts
|
* import { interval, take, lastValueFrom } from 'rxjs';
|
*
|
* async function execute() {
|
* const source$ = interval(2000).pipe(take(10));
|
* const finalNumber = await lastValueFrom(source$);
|
* console.log(`The final number is ${ finalNumber }`);
|
* }
|
*
|
* execute();
|
*
|
* // Expected output:
|
* // 'The final number is 9'
|
* ```
|
*
|
* @see {@link firstValueFrom}
|
*
|
* @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 lastValueFrom<T, D>(source: Observable<T>, config?: LastValueFromConfig<D>): Promise<T | D> {
|
const hasConfig = typeof config === 'object';
|
return new Promise<T | D>((resolve, reject) => {
|
let _hasValue = false;
|
let _value: T;
|
source.subscribe({
|
next: (value) => {
|
_value = value;
|
_hasValue = true;
|
},
|
error: reject,
|
complete: () => {
|
if (_hasValue) {
|
resolve(_value);
|
} else if (hasConfig) {
|
resolve(config!.defaultValue);
|
} else {
|
reject(new EmptyError());
|
}
|
},
|
});
|
});
|
}
|
