Ambient type for constructor parameter property with default does not include 'undefined' when the parameter is followed by one without a default
#48.547 geöffnet am 4. Apr. 2022
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Beschreibung
Bug Report
🔎 Search Terms
constructor parameter property default undefined
🕗 Version & Regression Information
This is the behaviour in every version in the Playground (3.3 through to 4.6.2)
⏯ Playground Link
Playground link with relevant code
💻 Code
class C { constructor(public a = 123, b: string) {} }
🙁 Actual behavior
.d.ts emit:
declare class C {
a: number;
constructor(a: number, b: string);
}
🙂 Expected behavior
Parameter a should be of type number | undefined. Otherwise, it's impossible to initialise the field to its default.
Note that if parameter b is removed, given a default, or made optional, a is then correctly emitted as optional (a?: number). Clearly TypeScript won't emit an optional followed by a required parameter, but it also doesn't add the undefined type.
If a is not a property (remove public) the emitted signature correctly includes undefined.
Secondary bug: undefined is also missing from the type hint for a. However, the compiler correctly accepts undefined when the class is declared inline:
That differs from the behaviour when only the .d.ts definition is available, where the compiler then rejects undefined in the same position.