strictNullChecks does not detect assertions done before property access

[connium]

TypeScript Version: 2.1.5

Code
In my jasmine tests I use .toBeDefined(...) to check whether the tested value is filled properly.

interface Person {
  name: string;
  address?: Address;
}

interface PersonService {
  getByName(name: string): Person;
}

it('should work as expected', () => {
  const person = personService.getByName('Norris');

  expect(person.address).toBeDefined();
  expect(person.address.state).toEqual('Oklahoma');
});

Running the compiler with strictNullChecks enabled results in a error TS2532: Object is possibly 'undefined'. on the line where person.address.state is accessed.

Checking explicitly for undefined resolves this issue:

it('should work as expected', () => {
  const person = personService.getByName('Norris');

  if (person.address === undefined) {
    throw new Error('address is undefined');
  }
  expect(person.address.state).toEqual('Oklahoma');
});

As this is a common pattern in my code, I thought it would be a good idea to extract the check in order to keep the tests clean and simple.

function assertNotUndefined(value: any): void|never {
  if (value === undefined) {
    throw new Error('value is undefined');
  }
}

it('should work as expected', () => {
  const person = personService.getByName('Norris');

  assertNotUndefined(person.address);
  expect(person.address.state).toEqual('Oklahoma');
});

Now I get the compiler error again.

Expected behavior:
The compiler does not complain about access to an potentially undefined variable when --strictNullChecks is set.

Actual behavior:
The compiler says error TS2532: Object is possibly 'undefined'.

[RyanCavanaugh]

See #9693

A pattern you can use is:

function assertNotUndefined<T>(value: T | undefined): value is T{
  if (value === undefined) {
    throw new Error('value is undefined');
    }
  return true;
}

it('should work as expected', () => {
  const person: {address?: {state: string}} = personService.getByName('Norris');

  if (!assertNotUndefined(person.address)) return;
  expect(person.address.state).toEqual('Oklahoma');
});

[connium]

You are right, my issue is similar to the one described in #9693. Didn't find it when looking for solutions to my problem somehow.

I like the idea of type guards as they are proposed in #8655. So closing this in favor of #8655.