mixin properties not checked

TypeScript Version: master (as of #26283)

Code

/**
 * @template {new (...args: any[]) => any} T
 * @param {T} SuperClass
 */
function myMixin(SuperClass) {
    return class extends SuperClass {
        method() { return 0; }
    }
};

class Base {}
const Mixed = myMixin(Base);

const instance = new Mixed();
instance.method(); // Correctly typed in signature help
instance.meshod(); // But not checked

Run with allowJs, checkJs, and strict.

Expected behavior:

Error at meshod.

Actual behavior:

No error.

This is true of Typescript as well:

function mix<T extends new (...args: any[]) => any>(S: T) {
    return class extends S {
        f() { return 0; }
    }
};

class Base {}
const Mixed = mix(Base);

export const m = new Mixed();
m.f(); // Correctly typed in signature help
m.j(); // should error here, but doesn't

Well, the reason that you see the apparent type of T is that the any comes from class extends S. S has type T, whose apparent type has a constructor with return type any, which is equivalent to extending any. That’s what allows you to access any property on m.

I though about using InstanceType in the return, that actually makes sense, although the code does throw when expected, it also throws extending S

function mix<T extends new (...args: any[]) => InstanceType<T>>(S: T) {
    return class extends S { // Throws: Base constructor return type 'InstanceType<T>' is not a class or interface type.
        f() { return 0; }
    }
};

class Base {}
const Mixed = mix(Base);

export const m = new Mixed();
m.f(); // Correctly typed in signature help
m.j(); // Throws: Property 'j' does not exist on type 'mix<typeof Base>.(Anonymous class) & Base'.

Maybe I'm using InstanceType poorly, but I think this should be the way to go.

Navigation Menu

Repository navigation

mixin properties not checked #26286

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

mixin properties not checked #26286

Description

Activity

sandersn commented on Aug 10, 2018

sandersn commented on Aug 13, 2018

michaeljota commented on Sep 11, 2018

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions