TypeScript's built-in mixins don't allow the mixin to have a useful constructor. I came across https://github.com/Microsoft/TypeScript/issues/14126#issuecomment-503940323 and wanted to check that it really works. It does!

oh-wow-mixins.ts

type ComposableConstructor<T, U> =
    [T, U] extends [new (a: infer O1) => infer R1,new (a: infer O2) => infer R2] ? {
        new (o: O1 & O2): R1 & R2
    } & Pick<T, keyof T> & Pick<U, keyof U> : never

function MixA<T extends new (o: any) => any>(Base: T) {
    class MixA extends (Base as new (...a: any[]) => {}) {
        a_value: number;
        constructor(options: { a_value: number }) {
            super(options)
            this.a_value = options.a_value;
        }
        a() {
          console.log("a: " + this.a_value)
        }
    }
    return MixA as ComposableConstructor<typeof MixA, T>
}

function MixB<T extends new (o: any) => any>(Base: T) {
    class MixB extends (Base as new (...a: any[]) => {}) {
        b_value: string;
        constructor(options: { b_value: string }) {
            super(options)
            this.b_value = options.b_value;
        }
        b() {
          console.log("b: " + this.b_value)
        }
    }

    return MixB as ComposableConstructor<typeof MixB, T>
}

class AB extends MixA(MixB(class { }))  {
    ab_value: string;
    constructor(options: { a_value: number, b_value: string, ab_value: string }) {
        super(options)
        this.ab_value = options.ab_value;
    }
    ab() {
      console.log("ab: " + this.ab_value)
    }
}
  
function main() {
  let ab = new AB({ a_value: 77, b_value: "wow", ab_value: "wowwww" });
  ab.a();
  ab.b();
  ab.ab()
}