Interface extension with omit produce incoherent results: Omit<C | D, 'a'> !== Omit<C, 'a'> | Omit<D, 'a'>
See original GitHub issueBug Report
๐ Search Terms
wrong set theory omit
๐ Version & Regression Information
ts-node-dev ver. 1.1.1 (using ts-node ver. 9.1.1, typescript ver. 4.1.3)
- This is the behavior in every version I tried, and I reviewed the FAQ for entries about it
- ts-node-dev ver. 1.1.1 (using ts-node ver. 9.1.1, typescript ver. 4.1.3)
- Nightly
โฏ Playground Link
Playground link with relevant code
๐ป Code
interface Basic {
a: string;
b: number;
}
interface BasicPlusC extends Basic {
c: string;
}
interface BasicPlusD extends Basic {
d: string;
}
type BasicPlusCOrD = BasicPlusC | BasicPlusD;
type BasicPlusCOrDMinusA = Omit<BasicPlusCOrD, 'a'>;
const getUnknown = (): BasicPlusCOrDMinusA => (
Math.random() > .5 ?
{ b: 0, c: ""} // <-- Causes error
: { b: 0, d: ""}
);
const getUnknownBis = (): Omit<BasicPlusC, 'a'> | Omit<BasicPlusD, 'a'> => (
Math.random() > .5 ?
{ b: 0, c: ""}
: { b: 0, d: ""}
);
const main = (): void => {
getUnknown();
getUnknownBis();
};
main();
๐ Actual behavior
Running this code causes the following error:
[INFO] 21:02:26 ts-node-dev ver. 1.1.1 (using ts-node ver. 9.1.1, typescript ver. 4.1.3)
Compilation error in /home/fabio/Projects/aalto/fullstack_open_2020_part9/part_c_patientor/backend/src/delete_me.ts
[ERROR] 21:02:27 โจฏ Unable to compile TypeScript:
src/delete_me.ts:20:13 - error TS2322: Type '{ b: number; c: string; } | { b: number; d: string; }' is not assignable to type 'Pick<BasicPlusCOrD, "b">'.
Type '{ b: number; c: string; }' is not assignable to type 'Pick<BasicPlusCOrD, "b">'.
Object literal may only specify known properties, and 'c' does not exist in type 'Pick<BasicPlusCOrD, "b">'.
20 { b: 0, c: ""} // <-- Causes error
~~~~~
๐ Expected behavior
I would have expected both functions, getUnknown
and getUnknownBis
to work correctly. This is because Iโm expecting the typing system to follow the same rules as set theory. In this case I have one function, getUnknown
, whose return type structure is ({ a, b, c } | ({ a, b, d, }) \ { a })
which means that the resulting structure should be in the form { b, c } | { b, d }
.
In the other function, getUnknownBis
, the return type structure is ({ a, b, c } \ { a }) | ({ a, b, d, }) \ { a })
which means that the resulting structure should be again in the form { b, c } | { b, d }
.
Hence I would expect both to work in the same way, but it looks like the first function is only expecting to return { b }
, instead of { b, c } | { b, d }
. This makes me think that the type inference engine is doing { b, c } & { b, d }
instead.
Issue Analytics
- State:
- Created 3 years ago
- Comments:5 (2 by maintainers)
For reference, using the suggested workaround by @IllusionMH, it works as I intended:
Still, I suggest updating the docs to mention this unexpected behaviour and the workaround as well (or even include it in the language with a different name, like UnionOmit, or something alike)
Thanks a lot, Iโm checking how the workaround works ๐ ๐