Add a new type of class declaration to support mixins

[AFatNiBBa]

🔍 Search Terms

"mixin", "InstanceType", "typeof", "class"

✅ Viability Checklist

• [x] This wouldn't be a breaking change in existing TypeScript/JavaScript code
• [x] This wouldn't change the runtime behavior of existing JavaScript code
• [x] This could be implemented without emitting different JS based on the types of the expressions
• [x] This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• [x] This isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
• [x] This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

When I run a mixin function, it outputs just the value of the returned expression, not the type

function myMixin<T extends new (...args: any[]) => any>(ctor: T) {
    return class extends ctor {
        a = 1;
    }
}

class MyClass {
    b = 2
}

const Result = myMixin(MyClass);

//             ↓ Result refers to a value, but is being used as a type here. Did you mean typeof Result ?          ↓ 
const istance: Result = new Result();

So I would need to define the type too

// ...
const Result = myMixin(MyClass);
type Result = InstanceType<typeof Result>;
// ...

I suggest adding a new syntax to define both a variable and a type with the same name

// ...
const Result = myMixin(MyClass) as type; // "as class" is also a viable option
// ...

Examples:

namespace Something {
    export class Idk {
        c = 1;
    }
}

const a = Something.Idk as type; // Ok

function someFunc() {
    return class {
        e = 1;
    }
}

const b = someFunc() as type; // Ok
const c = 1 as type; // Error: Expression "1" doesn't have any type attached

⭐ OPTIONAL extra generalised suggestion

Additionally, it would be nice if we could be able to generalise this process for all variables that also have an attached type

const a = 1;
type a = 2; // The type is unrelated to the value, no usage of this comes to my mind at the moment, but it already works

const b = a;
type b = a;
// Or
const b = a as type; // Defines a value from the `a` variable and a type from the `a` type

But it wouldn't work out of the box for the main suggestion, since Result doesn't actually have any type attached it would require functions (myMixin() for example) to be able to have attached return types

📃 Motivating Example

(Mentioned on the suggestion body)

💻 Use Cases

What do you want to use this for?

To improve the development of mixins in general

What shortcomings exist with current approaches?

Too verbose and feels like something it should to by default

const Result = myMixin(MyClass);
type Result = InstanceType<typeof Result>;

Creates an actual runtime class, that surely adds an overhead, althought it's minimal

class Result extends myMixin(MyClass) { }

What workarounds are you using in the meantime?

The runtime class one