Perform a side effect for every emission on the source Observable, but return
an Observable that is identical to the source.
tap<T>(nextOrObserver?: PartialObserver<T> | ((x: T) => void), error?: (e: any) => void, complete?: () => void): MonoTypeOperatorFunction<T>
Parameters
| nextOrObserver |
Optional. Default is undefined.
A normal Observer object or a
callback for next.
|
| error |
Optional. Default is undefined.
Callback for errors in the source.
|
| complete |
Optional. Default is undefined.
Callback for the completion of the source.
|
Returns
MonoTypeOperatorFunction<T>: An Observable identical to the source, but runs the
specified Observer or callback(s) for each item.
Description
Intercepts each emission on the source and runs a
function, but returns an output which is identical to the source as long as errors don't occur.
Returns a mirrored Observable of the source Observable, but modified so that
the provided Observer is called to perform a side effect for every value,
error, and completion emitted by the source. Any errors that are thrown in
the aforementioned Observer or handlers are safely sent down the error path
of the output Observable.
This operator is useful for debugging your Observables for the correct values
or performing other side effects.
Note: this is different to a subscribe on the Observable. If the Observable
returned by tap is not subscribed, the side effects specified by the
Observer will never happen. tap therefore simply spies on existing
execution, it does not trigger an execution to happen like subscribe does.
Example
Map every click to the clientX position of that click, while also logging the click event
import { fromEvent } from 'rxjs';
import { tap, map } from 'rxjs/operators';
const clicks = fromEvent(document, 'click');
const positions = clicks.pipe(
tap(ev => console.log(ev)),
map(ev => ev.clientX),
);
positions.subscribe(x => console.log(x));
Overloads
tap(next: null | undefined, error: null | undefined, complete: () => void): MonoTypeOperatorFunction<T>
Parameters
| next |
Type: null | undefined.
|
| error |
Type: null | undefined.
|
| complete |
Type: () => void.
|
Returns
MonoTypeOperatorFunction<T>
|
tap(next: null | undefined, error: (error: any) => void, complete?: () => void): MonoTypeOperatorFunction<T>
Parameters
| next |
Type: null | undefined.
|
| error |
Type: (error: any) => void.
|
| complete |
Optional. Default is undefined.
Type: () => void.
|
Returns
MonoTypeOperatorFunction<T>
|
tap(next: (value: T) => void, error: null | undefined, complete: () => void): MonoTypeOperatorFunction<T>
Parameters
| next |
Type: (value: T) => void.
|
| error |
Type: null | undefined.
|
| complete |
Type: () => void.
|
Returns
MonoTypeOperatorFunction<T>
|
tap(next?: (x: T) => void, error?: (e: any) => void, complete?: () => void): MonoTypeOperatorFunction<T>
Parameters
| next |
Optional. Default is undefined.
Type: (x: T) => void.
|
| error |
Optional. Default is undefined.
Type: (e: any) => void.
|
| complete |
Optional. Default is undefined.
Type: () => void.
|
Returns
MonoTypeOperatorFunction<T>
|
tap(observer: PartialObserver<T>): MonoTypeOperatorFunction<T>
Parameters
| observer |
Type: PartialObserver.
|
Returns
MonoTypeOperatorFunction<T>
|