Mock service provided to component using constructor-based DI

demo.component.ts

import { DemoService } from '../demo.service';
import { DemoComponent } from './demo.component';

describe('DemoComponent', () => {
  it('should create', () => {
    const serviceMock = { getWidgets: jest.fn() };
    const component = new DemoComponent(serviceMock as DemoService);
    expect(component).toBeTruthy();
  });
});