throwError

An Observable that emits an error immediately.

Signature

throwError(errorFactory: () => any): Observable

Marble diagram

throwError(...):  (#)

What it does

On each subscription it calls the error factory and sends the error via error. The factory ensures the stack trace is tied to the subscription site, not the Observable's creation site. The old form throwError(err) (without a factory) is deprecated.

When to use

In tests, simulating network failures. In catchError chains, converting one error into another (throwError(() => new MyAppError(...))). In switchMap, rejecting invalid inputs.

See also

catchError, retry, EMPTY