throwError

Just errors and does nothing else

This creation function is useful for creating an observable that will create an error and error every time it is subscribed to. Generally, inside of most operators when you might want to return an errored observable, this is unnecessary. In most cases, such as in the inner return of concatMap, mergeMap, defer, and many others, you can simply throw the error, and RxJS will pick that up and notify the consumer of the error.

Example

Create a simple observable that will create a new error with a timestamp and log it and the message every time you subscribe to it.

import { throwError } from 'rxjs';

let errorCount = 0;

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

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

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

// Logs the timestamp and a new error message each subscription;

Unnecessary usage

Using throwError inside of an operator or creation function with a callback, is usually not necessary:

import { throwError, timer, of } from 'rxjs';
import { concatMap } from 'rxjs/operators';

const delays$ = of(1000, 2000, Infinity, 3000);

delays$.pipe(
   concatMap(ms => {
     if (ms < 10000) {
       return timer(ms);
     } else {
       // This is probably overkill.
       return throwError(() => new Error(`Invalid time ${ms}`));
     }
   })
)
.subscribe({
   next: console.log,
   error: console.error
});

You can just throw the error instead:

import { throwError, timer, of } from 'rxjs';
import { concatMap } from 'rxjs/operators';

const delays$ = of(1000, 2000, Infinity, 3000);

delays$.pipe(
   concatMap(ms => {
     if (ms < 10000) {
       return timer(ms);
     } else {
       // Cleaner and easier to read for most folks.
       throw new Error(`Invalid time ${ms}`);
     }
   })
)
.subscribe({
   next: console.log,
   error: console.error
});