A mixin class must have a constructor with a single rest parameter of type any[] => this is wrong

[denis-migdal]

🔎 Search Terms

None

🕗 Version & Regression Information

Tested on the nightly build.

⏯ Playground Link

Playground Link

💻 Code

type Cstr<T> = {new(...args: any[]): T};

function mixins<T extends Cstr<any>>(klass: T) {

    return class Foo extends klass {

        readonly foo: number;

        constructor(foo: number, ...args: any[]) {
            super(...args);
            this.foo = foo;
        }
    }
}

🙁 Actual behavior

This raises the following error :

A mixin class must have a constructor with a single rest parameter of type any[]

🙂 Expected behavior

No TS errors should be raised.

Additional information about the issue

Instead of asking for a single rest parameters, TS should ask the last parameters to be a rest parameters.

Also, when #59812, is fixed, it should also be possible to do more complex constructor signatures, without relying on any[] :

type Cstr<T> = {new(...args: any[]): T};

type InferCstrParams<T> = T extends {new(...args: infer U): T} ? U : never;

function mixins<T extends Cstr<any>>(klass: T) {

    return class Foo extends klass {

        readonly foo: number;

        constructor(foo: number, ...args: InferCstrParams<T>) {
            super(...args);
            this.foo = foo;
        }
    }
}

Indeed, the constraints should be on the super() call, NOT on the mixins constructor.

This would also enable to generate the constructor parameters when the mixins e.g. has a type restrictions on klass, e.g. :

type Cstr<T, U extends any[]> = {new(...args: U): T};
function mixins<T extends Cstr2<any,[number]>>(klass: T) {

    return class Foo extends klass {
        constructor(a: number, b: number) {
            super(a+b);
        }
    }


}

This would also enable to create mixins taking many classes as parameters, etc.

[jcalz]

You didn’t provide any search terms. Did you search for existing issues before filing this?
If you search for mixin constructor in:title there are lots of hits.

Duplicate #37143

[denis-migdal]

From #14126

It seems a simple cast removes the error :

return class Foo extends (klass as { new(...args:[]): any }) {
    constructor(foo: number, b: boolean) { /* ... */ }
}

Note: if we want to add the base class parameters to the mixins class, we need to do another cast, cf the issue.

Playground Link

Seriously, why, for such simple code, are we forced to do such black magic found only in the darkness of the git issues ?

[typescript-bot]

This issue has been marked as "Question" and has seen no recent activity. It has been automatically closed for house-keeping purposes.