How to throw error from RxJS map operator (angular)

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Just throw the error inside the map() operator. All callbacks in RxJS are wrapped with try-catch blocks so it’ll be caught and then sent as an error notification.

This means you don’t return anything and just throw the error:

map(res => { 
  if (res.bearerToken) {
    return this.saveJwt(res.bearerToken); 
  } else {
    throw new Error('Valid token not returned');
  }
})

The throwError() (former Observable.throw() in RxJS 5) is an Observable that just sends an error notification but map() doesn’t care what you return. Even if you return an Observable from map() it’ll be passed as next notification.

Last thing, you probably don’t need to use .catchError() (former catch() in RxJS 5). If you need to perform any side effects when an error happens it’s better to use tap(null, err => console.log(err)) (former do() in RxJS 5) for example.

Jan 2019: Updated for RxJS 6

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