All Optional Object Interface Converts to any

[kitsonk]

I couldn't find a relevant issue, but this does seem rather fundamental and against the principle of:

1. Statically identify constructs that are likely to be errors.

TypeScript Version:

1.8.7

Code

interface Optional {
    foo?: string;
}

function foo(o: Optional): void {}

foo('bar'); // should throw
foo(1); // should throw
foo({ bar: 'baz' }); // should (and does) throw

Expected behavior:

Expected behaviour is that you should be able to type guard against other non-objects and only accept Objects with no properties or declared properties.

Actual behavior:

Passing anything other than an object literal that contains extra properties is acceptable.

[danquirk]

#3842 seems relevant

[RyanCavanaugh]

We talked about this briefly and think the idea in #3842 is worth trying out. I ported the PR forward but the code was written incorrectly around this specific case:

interface NotAllOptional {
  x: string;
}
interface AllOptional {
 y?: number;
}
type mixed= NotAllOptional & AllOptional;
let z: mixed = { x: 'ok' };

The code I had failed because we check intersection assignability by seeing if the source is assignable to each constituent in sequence; the first type succeeds and the second type fails (even though it shouldn't). A more complex check is needed here but I don't have time to implement it right now.

If someone wants to figure it out and send an updated PR that we could try against our internal suite of partner code, that'd be great. No guarantees it's going to be worth taking a breaking change over, but seeing what kind of issues (and non-issues) it finds might help us down the path to a more complete solution if needed.

[JabX]

I'm a bit confused, you just said that when you check assignability in an intersection you check the assignability of the source to both constituents, but if I have something like that:

interface A { a: string }
interface B { b: string }

const test: A & B
test = {a: 'a', b: 'b'}

It works fine (as expected) even though test isn't assignable to either A or B?

[JabX]

What's the status on this? In this something we can reasonably expect to be part of the 2.0 release?

I've been continously porting my app to Typescript over the past month or two and this is the one feature I desperately miss, as I'm using a lot of weak-typed objects to represent the state of a React component or for DTOs.

[kitsonk]

What's the status on this? In this something we can reasonably expect to be part of the 2.0 release?

The labels indicate that it a potentially good idea (Suggestion), but that the TypeScript team have limited resources and do not feel it is important enough to address (Accepting PRs) and are currently leaving it up to the community to address (Milestone = Community).