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Description
🔎 Search Terms
optional parameter properties, isolated declarations, TS9025: Declaration emit for this parameter requires implicitly adding undefined to its type. This is not supported with --isolatedDeclarations
🕗 Version & Regression Information
- This was present since the introduction of isolatedDeclarations in TS5.5
⏯ Playground Link
💻 Code
type MaybeUndefined = number | undefined;
type NeverUndefined = number;
export class Foo {
constructor(readonly x?: MaybeUndefined, readonly y?: NeverUndefined) {}
}
🙁 Actual behavior
Gives an error:
Declaration emit for this parameter requires implicitly adding undefined to its type. This is not supported with --isolatedDeclarations.
🙂 Expected behavior
It seems like this could be safely supported by just always emitting the class fields with a type of |undefined
whether that was redundant or not, something like:
export declare class Foo {
readonly x?: MaybeUndefined | undefined;
readonly y?: NeverUndefined | undefined;
constructor(x?: MaybeUndefined, y?: NeverUndefined);
}
Additional information about the issue
The workaround of explicitly adding |undefined
to the parameter property types is not too onerous, but still seems like an unnecessary restriction for isolatedDeclarations
mode.