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throwError

Creates an Observable that will create an error instance and push it to the consumer as an error immediately upon subscription.

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Import

import { throwError } from 'rxjs';

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Type Signature

function throwError(errorFactory: () => any): Observable<never>;

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Parameters

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errorFactory
() => any
required
A factory function that returns the error to emit. Called on each subscription.

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Returns

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Observable
Observable<never>
An Observable that errors immediately upon subscription.

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Description

Creates an Observable that errors immediately. Useful for error handling and testing.

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Examples

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Basic Error

import { throwError } from 'rxjs';

throwError(() => new Error('Oops!')).subscribe({
  error: err => console.error('Error:', err.message)
});

// Output:
// Error: Oops!

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Dynamic Error

import { throwError } from 'rxjs';

let errorCount = 0;

const error$ = throwError(() => {
  const error: any = new Error(`Error #${++errorCount}`);
  error.timestamp = Date.now();
  return error;
});

error$.subscribe({ error: err => console.log(err.message) });
error$.subscribe({ error: err => console.log(err.message) });

// Output:
// Error #1
// Error #2

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Common Use Cases

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Conditional Error

import { throwError, of } from 'rxjs';

function getUser(id: string) {
  if (!id) {
    return throwError(() => new Error('User ID required'));
  }
  return of({ id, name: 'User' });
}

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Testing Error Paths

import { throwError } from 'rxjs';
import { catchError, of } from 'rxjs';

throwError(() => new Error('Test error')).pipe(
  catchError(err => {
    console.error('Caught:', err.message);
    return of('Fallback');
  })
).subscribe(value => console.log(value));

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Important Note

Often you can just throw an error in operators instead of using throwError. RxJS will catch it and emit as an error.
  • of - Emit values successfully
  • defer - Lazy Observable creation
  • EMPTY - Observable that completes immediately

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See Also