Because in unit tests you want to test the component mostly isolated from other parts of your application, Angular won’t add your module’s dependencies like components, services, etc. by default. So you need to do that manually in your tests. Basically, you have two options here:
A) Declare the original NavComponent in the test
describe('AppComponent', () => {
beforeEach(async(() => {
TestBed.configureTestingModule({
declarations: [
AppComponent,
NavComponent
]
}).compileComponents();
}));
B) Mock the NavComponent
describe('AppComponent', () => {
beforeEach(async(() => {
TestBed.configureTestingModule({
declarations: [
AppComponent,
MockNavComponent
]
}).compileComponents();
}));
// it(...) test cases
});
@Component({
selector: 'app-nav',
template: ''
})
class MockNavComponent {
}
You’ll find more information in the official documentation.