RxJS: EmptyError: no elements in sequence (firstValueFrom / lastValueFrom on an empty Observable)

Meaning

These helpers reject with EmptyError when the source completes empty (an empty array in from([]), a filter that dropped everything, an HTTP call returning EMPTY). Provide a default ({ defaultValue: null }), use take(1) with defaultIfEmpty, or handle the empty case explicitly.

Common causes

• Source completed without emitting (filtered out, empty list, EMPTY)
• firstValueFrom used on a Subject that completes first
• first() after a filter that matches nothing
• Error handling returning EMPTY
• Route guard or resolver relying on a value that never arrives

Detailed fix by platform

Code examples

import { defaultIfEmpty, firstValueFrom } from 'rxjs';

const first = await firstValueFrom(this.items$.pipe(defaultIfEmpty(null)));   // null instead of EmptyError

How to diagnose

1. Source — Does the Observable emit before completing?
2. Operators — filter/EMPTY upstream?
3. Helper — Is a default value provided?

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