Property 'filter' does not exist on type 'Observable<Event>'

Question

Hi I am using Angular 2 final with router 3.0. I want to filter the events that are emitted from this.router.events

What I want to do :

import 'rxjs/operator/filter';

//...

this.router.events
  .filter((event:Event) => event instanceof NavigationEnd)
  .subscribe(x => console.log(x))

event can be instanceOf NavigationEnd, NavigationStart or RoutesRecognized but I want only NavigationEnd. But I get an error that

Property 'filter' does not exist on type Observable<Event>

during compile time.

When i import the whole rxjs library the error disappears. What should I import to make it work without loading the full rxjs library ?

Answer 1

UPDATE

For RXJS 5.x version:

import 'rxjs/add/operator/filter';

For RXJS 6.x version:

import { filter } from 'rxjs/operators';

The following rules have been designed by the RxJS team to help JavaScript developers refactor import paths:

1. rxjs/operators: Contains all pipeable operators.

import { map, filter, scan } from 'rxjs/operators';

2. rxjs: Contains creation methods, types, schedulers, and utilities.

import { Observable, Subject, asapScheduler, pipe, of, from, interval, merge, fromEvent } from 'rxjs';

Answer 2

There are several possible fixes for this scenario.

1) Use pipeable operators

Pipeable operators are meant to be a better approach for pulling in just the operators you need than the "patch" operators found in rxjs/add/operator/*

import { filter } from 'rxjs/operators';

// ..

 this.router.events.pipe(
   filter((event:Event) => event instanceof NavigationEnd)
 ).subscribe(x => console.log(x))

---

2) Use 'rxjs/add/operator/filter'

Change the import statement to import 'rxjs/add/operator/filter'. This will modify Observable.prototype and add filter method to an every instance of the Observable class.

There are two consequences:

• it is enough to execute the import statement just once per the application
• in a shared library/npm package this might bring some confusion to a library consumer (filter() method will magically appear under Observable while using the library)

---

3) Leave the operator import but change how it is called

The statement import 'rxjs/operator/filter' is perfectly valid. It will import just the operator. This approach will not mess with the Observable.prototype. On downside it will make it more difficult to chain several operators.

import 'rxjs/operator/filter'; // This is valid import statement.
                               // It will import the operator without 
                               // modifying Observable prototype
// ..

// Change how the operator is called
filter.call(
   this.router.events, 
   (event:Event) => event instanceof NavigationEnd
).subscribe(x => console.log(x));

---

More details: Pipeable Operators

Answer 3

Angular Update(5.x to 6.x) also comes with update of rxjs from 5.x to 6.x So simply add

import { filter } from 'rxjs/operators';

then

this.router.events.pipe(
  filter((event:Event) => event instanceof NavigationEnd)
).subscribe(x => console.log(x))

Hope That helps someone