Erroneous "Object is possibly null" message

[akanix42]

TypeScript Version: 2.2.1

The option strictNullChecks is enabled.

Code

// A *self-contained* demonstration of the problem follows...
class Foo {
  public foo: string | null;

  bar() {
    const shouldDoSomething = this.foo !== null;

    if (shouldDoSomething) {
      return this.foo.length;
    }
  }
}

Expected behavior:
There should not be an error, since I have just checked that the object isn't null. This should have the same effect as the following:

class Foo {
  public foo: string | null;

  bar() {
    if (this.foo !== null) {
      return this.foo.length;
    }
  }
}

Actual behavior:
TypeScript throws an error on the foo object: "TS2531 Object is possibly null".

The workaround I'm using follows, but isn't what I'd consider ideal:

class Foo {
  public foo: string | null;

  bar() {
    const shouldDoSomething = this.foo !== null;

    if (shouldDoSomething) {
      return (<string>this.foo).length;
    }
  }
}

As a real example, I have a boolean shouldEmitToOccupant that determines whether or not an event should be emitted to an occupant of a certain tile in my game. This variable is only true if several tests are passed, the first of which is the null check. For now I'm using an assertion, but it seems like I shouldn't have to...

[RyanCavanaugh]

Same response as #10830. #10421 would be a good solution.

What I usually do is move out the null check to an early return:

class Foo {
  public foo: string | null;

  bar() {
    if (this.foo === null) return;
    const shouldDoSomething = (more checks...);

    if (shouldDoSomething) {
      return this.foo.length;
    }
  }
}

[aluanhaddad]

Why not use a local const binding? If foo were a getter there would be no way to know that it would not return null the next time it was evaluated.

[RyanCavanaugh]

That wouldn't help since there's still no tracking between shouldDoSomething and the local const's nullness

[aluanhaddad]

@RyanCavanaugh gotcha, thanks for explaining.