Mix methods into a type

Mixin.ts

type AbstractConstructorHelper<T> = (new (...args: any) => {}) & T;

type Extension = Record<string, {[key in 'method']: unknown}>

type Schematic<T extends Extension> = {
  -readonly [K in keyof T]: T[K]['method']
}

type Mixin<T, S extends Extension> = (new (...args: ConstructorParameters<AbstractConstructorHelper<T>>) => InstanceType<AbstractConstructorHelper<T>> & Schematic<S>) & T;

function mixin<T, S extends Extension>(target: T, methods: S): Mixin<T, S> {
    // Logic to mix methods in target goes here
    
    return (target as unknown) as (Mixin<T, S>)
}

class Geometry {
    width: number = 0
    height: number = 0
    
    constructor(width: number, height: number) {
        this.width = width
        this.height = height
    }
}

const methods = {
    square: {
        method(this: Geometry): number {
            return this.width * this.height
        },
    }
}

const Rectangle = mixin(Geometry, methods)

const rect = new Rectangle(100, 200)

console.log(rect.square())

AbstractConstructorHelper got from this comment: microsoft/TypeScript#31278 (comment)