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Multiple keys being remapped to same key causes unstable modifier propagation #62318
Description
🔎 Search Terms
Mapped Types, Key Remapping
🕗 Version & Regression Information
- This has been happening since first release of key remapping at 4.1 (tested with v4.1.5 at Playground) until nightly (tested with v6.0.0-dev.20250822 at Playground).
⏯ Playground Link
💻 Code
type RemapKeyToInitialPart<T> = {
[K in keyof T as K extends `${infer First}.${infer _Rest}` ? First : K]: null;
};
type FirstOptional = RemapKeyToInitialPart<{
// ^?
"foo.bar"?: string;
"foo.baz": number;
}>;
type FirstRequired = RemapKeyToInitialPart<{
// ^?
"foo.baz": number;
"foo.bar"?: string;
}>;🙁 Actual behavior
type FirstOptional and type FirstRequired results in different types:
type FirstOptionalhas optional propertyfootype FirstRequiredhas required propertyfoo
🙂 Expected behavior
type FirstOptional and type FirstRequired results in same type. I would expect it to be same as type FirstRequiredtype FirstOptional.
Additional information about the issue
My motivation was to implement a utility type Expand<T> which transforms { "foo.bar": number } to { foo: { bar: number } } (playground)
Activity
jcalz commentedon Aug 23, 2025
I agree that you want the output not to depend on the order of properties in the input, since order of properties is supposed to be unobservable.
But I don't see why you expect the property to be required. When you collapse multiple input properties to a single property with key remapping you get the union of the property types, e.g:
Playground link
So I'd expect to see the sort of thing you get when you combine unions of optional-and-required, which is effectively optional (you can think of
| undefinedas being sort of optional, and so if any one of the inputs is optional and included| undefined, the output should also).cm-ayf commentedon Aug 23, 2025
Joe Calzaretta (@jcalz) Thank you for the comment! I've changed the description.
By the way, the selection would be a sort of design decision, so my (stronger) expectation is that TypeScript team decide whether it should or should not be optional.
RyanCavanaugh commentedon Aug 29, 2025
Discussed at design meeting:
readonlyFixed mapped type modifier propagation when remapping multiple keys t…