How to unit test function called in service's constructor?

Question

How can I test, in the service's spec file, that a function is called in the constructor? For example:

@Injectable({
    providedIn: 'root'
})
export class myService {
  

    constructor() {
       this.myFunction();
    }

    myFunction(){}
}

So how can I test that my function was called?

beforeEach(() => {
TestBed.configureTestingModule({});
    service = TestBed.get(myService);

I can't spy on the service before the testbed.get, and I tried:

it('should demonstrate myFunction called in constructor', () => {
  const spy = spyOn (myService, 'myFunction');
  const serv = new myService();

  expect(spy).toHaveBeenCalled();
});

But this is failing to say spy wasn't called!

Any help would be most appreciated.

Answer 1

Use spyOn(obj, methodName) → {Spy} to spy the myFunction on MyService.prototype.

E.g.

service.ts:

import { Injectable } from '@angular/core';

@Injectable({
  providedIn: 'root',
})
export class MyService {
  constructor() {
    this.myFunction();
  }

  myFunction() {}
}

service.test.ts:

import { MyService } from './service';

describe('63819030', () => {
  it('should pass', () => {
    const myFunctionSpy = spyOn(MyService.prototype, 'myFunction').and.stub();
    const service = new MyService();
    expect(myFunctionSpy).toHaveBeenCalledTimes(1);
  });
});