can't test condition for non-existed key in object about jest HOT 7 CLOSED

describe('footest', function() {
  it('checks the foo', function() {
    var x = {};
    console.log(x.y === undefined);
  });
});

Using Jest CLI v0.1.15
Found 1 matching tests...
 PASS  __tests__/foo-test.js (0.012s)
true
1 tests passed (1 total)
Run time: 0.71s

➜  example-react git:(master) ✗ npm test

> [email protected] test /tmp/example-react
> jest

Found 1 matching tests...
 FAIL  __tests__/my_component_test.coffee (0.487s)
● TalkBox › it has a video element
  - Error: Invariant Violation: mergeObjectsWithNoDuplicateKeys(): Tried to merge two objects with the same key: room
        at invariant (/tmp/example-react/node_modules/react/lib/invariant.js:52:19)
        at /tmp/example-react/node_modules/react/lib/ReactCompositeComponent.js:548:46
        at objMap (/tmp/example-react/node_modules/react/lib/objMap.js:41:23)
        at mergeObjectsWithNoDuplicateKeys (/tmp/example-react/node_modules/react/lib/ReactCompositeComponent.js:547:3)
        at mergedResult [as getInitialState] (/tmp/example-react/node_modules/react/lib/ReactCompositeComponent.js:576:12)
        at ReactCompositeComponentMixin.mountComponent (/tmp/example-react/node_modules/react/lib/ReactCompositeComponent.js:926:48)
        at null.mountComponent (/tmp/example-react/node_modules/react/lib/ReactPerf.js:57:21)
        at ReactCompositeComponentMixin.mountComponent (/tmp/example-react/node_modules/react/lib/ReactCompositeComponent.js:952:44)
        at null.mountComponent (/tmp/example-react/node_modules/react/lib/ReactPerf.js:57:21)
        at ReactComponent.Mixin._mountComponentIntoNode (/tmp/example-react/node_modules/react/lib/ReactComponent.js:561:25)
        at ReactReconcileTransaction.Mixin.perform (/tmp/example-react/node_modules/react/lib/Transaction.js:159:20)
        at ReactComponent.Mixin.mountComponentIntoNode (/tmp/example-react/node_modules/react/lib/ReactComponent.js:537:19)
        at Object.ReactMount._renderNewRootComponent (/tmp/example-react/node_modules/react/lib/ReactMount.js:309:25)
        at Object._renderNewRootComponent (/tmp/example-react/node_modules/react/lib/ReactPerf.js:57:21)
        at Object.ReactMount.renderComponent (/tmp/example-react/node_modules/react/lib/ReactMount.js:359:32)
        at Object.renderComponent (/tmp/example-react/node_modules/react/lib/ReactPerf.js:57:21)
        at Object.ReactTestUtils.renderIntoDocument (/tmp/example-react/node_modules/react/lib/ReactTestUtils.js:57:18)
        at Spec.<anonymous> (/tmp/example-react/public/__tests__/my_component_test.coffee:47:15)
        at Timer.listOnTimeout [as ontimeout] (timers.js:110:15)
  - Error: Did not find exactly one match for tag:video
        at Object.ReactTestUtils.findRenderedDOMComponentWithTag (/tmp/example-react/node_modules/react/lib/ReactTestUtils.js:173:13)
        at Spec.<anonymous> (/tmp/example-react/public/__tests__/my_component_test.coffee:52:23)
        at Timer.listOnTimeout [as ontimeout] (timers.js:110:15)
1 tests failed, 0 tests passed (1 total)

➜  example-react git:(latest-firebase-simple-login) ✗ npm test

> [email protected] test /tmp/test/example-react
> jest

Found 1 matching tests...
 PASS  __tests__/my_component_test.coffee (0.495s)
1 tests passed (1 total)
Run time: 1.497s

undefined = 2;
console.log(typeof undefined);