1 Mar /Testing time based activities in Angular

Posted by Dom Miller

When testing time streams or promised based activities use ‘fakeAsync’ and ‘tick’.

For example, given a service:

import { Injectable } from '@angular/core';
import { Observable } from 'rxjs/Rx';

export class CountdownService {
  // counts down from given number to zero
  source(num: number): Observable<number> {
    return Observable.timer(1000, 1000)
    .map( (v) => num - v)
    .takeWhile( (v) => v !== -1);

To test the CountDown class source method:

// … test setup above, inside describe function

it('should count down from 5 to 0', inject([CountdownService], fakeAsync( (service: CountdownService) => {
    let num: number = undefined;
    const count = service.source(5).subscribe( (value) => num = value);


Wrap the ‘it’ callback function in fakeAsync() which creates a new zone for the asynchronous code to run.

Calling tick() with a time span in ms, simulates the code after that amount of time has passed.

Use tick() with no arguments for promise based code

My expectation is based on what happens after 7 seconds has passed.