throwError
Creates an observable that will create an error instance and push it to the consumer as an error immediately upon subscription.
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 ${ ++errorCount }`);
error.timestamp = Date.now();
return error;
});
errorWithTimestamp$.subscribe({
error: err => console.log(err.timestamp, err.message)
});
errorWithTimestamp$.subscribe({
error: err => console.log(err.timestamp, err.message)
});
// Logs the timestamp and a new error message for each subscription
Unnecessary usage
Using throwError
inside of an operator or creation function
with a callback, is usually not necessary
import { of, concatMap, timer, throwError } from 'rxjs';
const delays$ = 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 ${ ms }`));
}
})
)
.subscribe({
next: console.log,
error: console.error
});
You can just throw the error instead
import { of, concatMap, timer } from 'rxjs';
const delays$ = 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 ${ ms }`);
}
})
)
.subscribe({
next: console.log,
error: console.error
});
Creates an observable that will create an error instance and push it to the consumer as an error immediately upon subscription.
Just errors and does nothing else
This creation function is useful for creating an observable that will create an error and error every time it is subscribed to. Generally, inside of most operators when you might want to return an errored observable, this is unnecessary. In most cases, such as in the inner return of concatMap, mergeMap, defer, and many others, you can simply throw the error, and RxJS will pick that up and notify the consumer of the error.
Example
Create a simple observable that will create a new error with a timestamp and log it and the message every time you subscribe to it
import { throwError } from 'rxjs';
let errorCount = 0;
const errorWithTimestamp$ = throwError(() => {
const error: any = new Error(`This is error number ${ ++errorCount }`);
error.timestamp = Date.now();
return error;
});
errorWithTimestamp$.subscribe({
error: err => console.log(err.timestamp, err.message)
});
errorWithTimestamp$.subscribe({
error: err => console.log(err.timestamp, err.message)
});
// Logs the timestamp and a new error message for each subscription
Unnecessary usage
Using throwError
inside of an operator or creation function
with a callback, is usually not necessary
import { of, concatMap, timer, throwError } from 'rxjs';
const delays$ = 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 ${ ms }`));
}
})
)
.subscribe({
next: console.log,
error: console.error
});
You can just throw the error instead
import { of, concatMap, timer } from 'rxjs';
const delays$ = 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
});
§Parameters
§Return Type
Returns an observable that will error with the specified error immediately upon subscription.
§Return Type
Notifies the consumer of an error using a given scheduler by scheduling it at delay 0
upon subscription.
§Parameters
A scheduler to use to schedule the error notification