RxJS: TypeError: You provided 'undefined' where a stream was expected. You can provide an Observable, Promise, ReadableStream, Array, AsyncIterable, or Iterable.

Meaning

Operators like switchMap(x => this.load(x)) must return an Observable. If the inner function has curly braces without a return, or a service method is undefined (typo, injection missing), the operator gets undefined. Mocks in tests that return nothing cause the same error.

Common causes

• Arrow function with { } and no return inside switchMap / mergeMap
• Service method or property is undefined (typo, wrong injection)
• Test mock returns undefined instead of of(...)
• forkJoin array containing an undefined entry
• Conditional branch returning nothing

Detailed fix by platform

Code examples

this.route.paramMap.pipe(
  map(p => p.get('id')),
  switchMap(id => id ? this.api.load(id) : of(null)),   // both branches return an Observable
).subscribe(item => (this.item = item));

How to diagnose

1. Operator — Which operator raised it?
2. Return — Does the callback return an Observable?
3. Mock — Does the test double return one?

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