单元测试用例来检查函数是否被调用

Question

I'm trying to write a unit test case to check if the function is being called inside the switch case I have a following function

public onMessageReceived(event) {
    switch (event && event['event']) {
      case 'onOptionSelected':
        this.toggleNext();
        break;

      case 'temp':
        break;
    }
  }

and have tried to write the unit test case for the same as below ,but the test case always fails 'Expected spy toggleNext to have been called.' what could be issue any guidance would be appreciated.

 it('should call toggleNext on onOptionSelected event', () => {
    component.onMessageReceived({ event: 'OptionSelected' });
    fixture.detectChanges();
    fixture.whenStable().then(() => {
      expect(spyOn<any>(component, 'toggleNext')).toHaveBeenCalled();
    });
  });

Answer 1

Declare the spy first, before you call the function

 it('should call toggleNext on onOptionSelected event', () => {
    const toggleSpy = spyOn(component, 'toggleNext');
    component.onMessageReceived({ event: 'onOptionSelected' });
    expect(toggleSpy).toHaveBeenCalled()
  });