Description
Bug Report
I was working with a library which reported that a property had an additional value of undefined, and the author of the plugin claimed that the error went by removing readonly. After investigating I found the following, which I found confusing.
The framework we use (vue) has a function for marking objects as deeply readonly:
export declare function readonly<T extends object>(target: T): DeepReadonly<T>;
export declare type DeepReadonly<T> = T extends {} ? {
readonly [K in keyof T]: DeepReadonly<T[K]>;
} : Readonly<T>;When wrapping an object with optional properties:
export declare class User {
a?: string;
b: string;
}
export const user = readonly(new User() )... the exported types have an additional undefined at the type definition, even though it wasn't allowed in the original type.
export declare class User {
a?: string;
b: string;
}
export declare const user: {
readonly a?: string | undefined;
readonly b: string;
};🔎 Search Terms
extra undefined, nested readonly, deepreadonly
🕗 Version & Regression Information
- This is the behavior in every version I tried.
⏯ Playground Link
Playground link with relevant code
💻 Code
This was the smallest I could reduce the code to and still see the compiler doing the unexpected:
export declare function readonly<T extends object>(target: T): DeepReadonly<T>;
export declare type DeepReadonly<T> = T extends {} ? {
readonly [K in keyof T]: DeepReadonly<T[K]>;
} : Readonly<T>;
export declare class User {
a?: string;
b: string;
}
export declare type ReadOnlyUser = DeepReadonly<User>
export const user = readonly(new User() )🙁 Actual behavior
The code above is transformed to the following types. The generated type for the variable user is incompatible with the exported class User, because it adds undefined as type to the type of the optional parameter of the class.
export declare const user: {
readonly a?: string | undefined;
readonly b: string;
};🙂 Expected behavior
The types of User and user should be identical, the latter should just have all props as readonly, but there should be no other changes to the type.
export declare const user: {
readonly a?: string;
readonly b: string;
};