OneOf type for interface without discriminators

[realyze]

I have the following type

type OneOf<T, U> = { [P in keyof T]?: never } & U | { [P in keyof U]?: never } & T;

The idea is that I could use it when I have an interface that can have one combination of props or the other (e.g. size: number vs horizontalSize: number; verticalSize: number).

Now, I would think

const getSize = (props: Props) => typeof props.size === 'undefined` ? props.horizontalSize : props.size;

Would have type number but it actually has number | undefined.

Should TypeScript be able to determine that the other "branch" should be used (i.e., size is missing ergo horizontalSize must be defined)?

[mhegazy]

Not sure this type is what you want then. Looks like the type you are looking for is { size: number } | { horizontalSize: number; verticalSize: number; }.

Since the union does not have a discriminant (i.e. a common property with a unit type that is different in each constituent), the only way to narrow it is using in operator:

declare var foo: { size: number } | { horizontalSize: number; verticalSize: number; };

"size" in foo ? foo.size : foo.horizontalSize;

[mhegazy]

You might have better luck sharing this on StackOverflow. This is not a support forum, it is one for reporting compiler/language bugs and suggestions.
If you believe you are running into a compiler bug, please share a minimal repro project that demonstrates the issue you are running into and why you believe this is a compiler bug and not a declaration file issue or library usage problem.

[realyze]

@mhegazy Thanks for the tip! I didn't want to word the ticket as a bug because I wasn't sure whether that was expected TS behaviour or not; it definitely surprised me.

Using in works as a charm so I'll close this. Thanks again.

[realyze]

Actually, apologies for another ping, @mhegazy, but there is a Typescript bug in here I think which surfaces when I use the in syntax:

type Foo = {
  size: number
} | {
  horizontalSize: number;
  verticalSize: number;
};

const bar: Foo = {
  size: 42,
  horizontalSize: 43,
  verticalSize: 44,
};

In the above I would think bar should report an error as Foo is not be allowed to have all three props defined. But it actually compiles fine.

[realyze]

Looks like #20863 already tracks that (took me a while to find it).