angular2 unit test: cannot read property of componentInstance.method() undefined

The problem is that asynchronous beforeEach is incorrectly implemented, this results in race condition.

Doing .compileComponents().then(() => { ... }) in beforeEach block results in delaying code execution in then callback at least for one tick. it block never waits and accesses myComponent variable before it had a chance to be assigned.

This kind of race conditions can become less obvious and more dangerous when a test doesn't fail. Instead, tests can become cross-contaminated when beforeEach from previous tests affects variables in current test.

.compileComponents() is synchronous, unless there are components with styleUrls and templateUrl (like in the case above). In this case it becomes asynchronous, and async helper should be used:

// asynchronous block
beforeEach(async(() => {    
  TestBed.configureTestingModule({ ... })
  .compileComponents();
}));

// synchronous block
beforeEach(() => {    
  myComponent = ...
});

As a rule of thumb, blocks should be wrapped with async of fakeAsync helper if there's a chance that block can be asynchronous.

When component classes are tested with TestBed, they follow a lifecycle and their hooks are called automatically. Calling ngOnInit() manually is not needed (as another answer explains) and will result in calling the hook twice.

You don't have to call ngOnInit() manually to run the component's init().

Modify your code to below code

let myComponent: MyComponent;
let myService: MyService;
let fixture;

describe('myComponent', () => {
   beforeEach(() => {
    TestBed.configureTestingModule({
      declarations: [MyComponent],
    providers: [
      {provide: MyService, useClass: MockMyService} // **--passing Mock service**
  ]
}).compileComponents()
  .then(() => {
   fixture = TestBed.createComponent(MyComponent);
    myComponent = TestBed.createComponent(MyComponent).componentInstance;
    myService = TestBed.get(MyService);
    console.log(myService.getData());
  });
});

it('should get the mock data', () => {
   fixture.detectChanges();  // this line will call components ngOnInit() method
   expect(myComponent.data).toBe(DATA_OBJECT);
  });
})

Look at the line fixture.detectChanges(); First time when change detection happens components ngOnInit() will be called.