Angular service testing: Cannot find name 'asyncData'

I quote from: https://angular.io/guide/testing

The async observable was produced by an asyncData helper. The asyncData helper is a utility function that you'll have to write yourself. Or you can copy this one from the sample code:

testing/async-observable-helpers.ts

/** Create async observable that emits-once and completes
 *  after a JS engine turn */
export function asyncData<T>(data: T) {
  return defer(() => Promise.resolve(data));
}

Note: defer comes from rxjs, i.e.: import { defer } from 'rxjs';

Change this line:

httpClientSpy.get.and.returnValue(asyncData(expectedHeroes));

to use the Observable operator of()

httpClientSpy.get.and.returnValue(of(expectedHeroes));

This will return an observable that can be subscribed to and will return expectedHeroes. If you are using Angular 6, you can import this directly from rxjs:

import {of} from 'rxjs';

If download sample code from angular docs, you will find 'asyncData' definition in testing>async-observable-helpers.ts file.