How to check if observable countdown timer has completed?

I have a simple countdown timer in angular 4

import {Observable} from 'rxjs/Observable';
import 'rxjs/add/observable/timer';
import 'rxjs/add/operator/map';
import 'rxjs/add/operator/take';

countDown;
counter = 60;
enableButton:boolean = false;

this.countDown = Observable.timer(0,1000).take(this.counter).map(() => --
 this.counter);

I want to make enableButton boolean true when this observable has been completed.

How should I write that code? Please Help

Here is a plunker link, which I am using in my project https://plnkr.co/edit/TWMIEd1UykUdUC37qyFK?p=preview

How to check if observable countdown timer has completed?

3 Answers