Ambient type for constructor parameter property with default does not include 'undefined' when the parameter is followed by one without a default

[yseymour]

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.

[a-tarasyuk]

@RyanCavanaugh Should TS add undefined in strict/non-strict mode?

[RyanCavanaugh]

I would say both? Generally we prefer declaration emit to make strict-friendly output files even if the input project isn't strict