How to cancel current request in interceptor - Angular 4

Question

As you know it\'s possible to use Interceptors in new versions of Angular 4.

In mine, I want to cancel a request in interceptor in some conditions. So i

Answer 1

Inspired by @RVP answer I have found out that it's possible to cut the chain with an error in the same simple way using Observable.throw()

//...
intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
    if (stopThisRequestWithError) {
        return Observable.throw('Error message');
    } else {
        return next.handle(req);
    }
}

This avoids fulfilling response promises with undefined values.

Answer 2

As suggested above, there is more optimal way to handle the error with custom response

import { throwError, Observable } from 'rxjs';
import { HttpEvent } from '@angular/common/http';

intercept(request: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
  if (authFailedDummy(request)) {
    return throwError(new Error('Authorization error: The request did not go through'));
  }
  return next.handle(request);
}

Answer 3

This is just a slight variant of RVP's answer

import { NEVER } from 'rxjs';

intercept(request: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
  if (stopThisRequest) {
    return NEVER;
  }

  return next.handle(request);
}

I used NEVER instead of EMPTY to avoid dealing with undefined values in my subscriptions (or promises).

Use NEVER if you don't want your subscription callback to be invoked

Answer 4

let sub = this.http.get(url, {headers: reqHeaders})
            .subscribe(
                (res) => {
                    res = res.json();
                }
            );

sub.unsubscribe();

Answer 5

@RVP's code will work, We can do one more thing for same.

add return only, it will also work

    intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
          if(helper.isTokenExpired(tokenVal)){
              return;
          }
        .
        . <{code}>
        .

    }

Answer 6

To Angular 6, you need can user the following structure to return a empty Observable:

import {Observable} from 'rxjs';
import {empty} from 'rxjs/internal/Observer';

//...

intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
    if (stopThisRequest) {
        return Observable.create(empty);
    } else {
        return next.handle(req);
    }
}