catch.md

catch / catchError

signature: catchError(project : function): Observable

Gracefully handle errors in an observable sequence.

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⚠ Remember to return an observable from the catchError function!

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Examples

( example tests )

Example 1: Catching error from observable

( StackBlitz | jsBin | jsFiddle )

// RxJS v6+
import { throwError, of } from 'rxjs';
import { catchError } from 'rxjs/operators';
//emit error
const source = throwError('This is an error!');
//gracefully handle error, returning observable with error message
const example = source.pipe(catchError(val => of(`I caught: ${val}`)));
//output: 'I caught: This is an error'
const subscribe = example.subscribe(val => console.log(val));

Example 2: Catching rejected promise

( StackBlitz | jsBin | jsFiddle )

// RxJS v6+
import { timer, from, of } from 'rxjs';
import { mergeMap, catchError } from 'rxjs/operators';

//create promise that immediately rejects
const myBadPromise = () =>
  new Promise((resolve, reject) => reject('Rejected!'));
//emit single value after 1 second
const source = timer(1000);
//catch rejected promise, returning observable containing error message
const example = source.pipe(
  mergeMap(_ =>
    from(myBadPromise()).pipe(catchError(error => of(`Bad Promise: ${error}`)))
  )
);
//output: 'Bad Promise: Rejected'
const subscribe = example.subscribe(val => console.log(val));

Example 3: Catching errors comparison when using switchMap/mergeMap/concatMap/exhaustMap

( StackBlitz )

// switchMap in example below can be replaced with mergeMap/concatMap/exhaustMap, the same behaviour applies
import { throwError, fromEvent, of } from 'rxjs';
import {
  catchError,
  tap,
  switchMap,
  mergeMap,
  concatMap,
  exhaustMap
} from 'rxjs/operators';

const fakeRequest$ = of().pipe(
  tap(_ => console.log('fakeRequest')),
  throwError
);

const iWillContinueListening$ = fromEvent(
  document.getElementById('continued'),
  'click'
).pipe(
  switchMap(_ => fakeRequest$.pipe(catchError(_ => of('keep on clicking!!!'))))
);

const iWillStopListening$ = fromEvent(
  document.getElementById('stopped'),
  'click'
).pipe(
  switchMap(_ => fakeRequest$),
  catchError(_ => of('no more requests!!!'))
);

iWillContinueListening$.subscribe(console.log);
iWillStopListening$.subscribe(console.log);

Additional Resources

• catchError 📰 - Official docs
• catchError - In Depth Dev Reference
• Error handling operator: catch 🎥 💵 - André Staltz

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📁 Source Code: https://github.com/ReactiveX/rxjs/blob/master/src/internal/operators/catchError.ts