firstValueFrom

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.

firstValueFrom<T, D>(source: Observable<T>, config?: FirstValueFromConfig<D>): Promise<T | D>

Parameters

source	the observable to convert to a promise
config	Optional. Default is undefined. a configuration object to define the defaultValue to use if the source completes without emitting a value

Returns

Promise<T | D>

Description

If the observable stream completes before any values were emitted, the returned promise will reject with 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 timeout, take, takeWhile, or takeUntil amongst others.

Example

Wait for the first value from a stream and emit it from a promise in an async function.

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 Also

• lastValueFrom