Type Narrowing not being all it can be

[johnnyreilly]

TypeScript Version: 2.4.1

Hey all, I stumbled upon an oddity around type narrowing. I've been meaning to report for ages but... well better late than never!

Code

interface OneThing {
    oneThing: {};
}

interface OtherThing {
    otherThing: {};
}

const returnOneThingOrAnother = () =>
    '5' === `${6}` // ignore this - what's produced is what matters
        ? { oneThing: {} } as OneThing
        : { otherThing: {} } as OtherThing;

const iHaveOneThingOrAnother = returnOneThingOrAnother();

if (iHaveOneThingOrAnother.oneThing) { 
    // iHaveOneThingOrAnother should be narrowed to OneThing
} else {
    // iHaveOneThingOrAnother should be narrowed to OtherThing
}

Expected behavior:
I would expect type narrowing to occur in the manner suggested in the comments.

Actual behavior:
Actually the expected type narrowing does not occur. Rather, the if (iHaveOneThingOrAnother.oneThing) { expression throws the following exception:

Property 'oneThing' does not exist on type 'OneThing | OtherThing'.
  Property 'oneThing' does not exist on type 'OtherThing'.

Why? Obviously I could solve this by introducing a user defined type guard but that seems like it ought not to be necessary.

Is there a reason the TypeScript compiler does not perform the type narrowing in the way I had hoped?

Possibly related to #12457 but not certain.

Much ❤️ for all your work BTW 🌻

[RyanCavanaugh]

Good question. We consider this to be an unsafe operation because we can't guarantee something like this doesn't happen:

interface GotString {
  x: string;
}
interface GotNumber {
  y: number;
}
const fooled = { x: 100, y: 200 };
const gn: GotNumber = fooled;
const j: GotString | GotNumber = Math.random() > 0.5 ? gn : { x: 'ok' };
if (j.x) {
  // In theory: j: GotString
  j.x.substr(0); // Crash - x was a number. Bamboozled!
}

See also #14094 where it's proposed to add syntax to create these sorts of unions automatically.

[johnnyreilly]

Thanks for the response @RyanCavanaugh - that makes sense.

I'm quite keen on exclusive unions so at present I'm landed with either introducing a type guard or casting back to any briefly - neither of which is attractive. Suffice to say I like the look of #14094!

Do feel free to close this issue if you think it's covered by #14094. Good to know the reason for this 👍