typescript-generics.ts

interface Serializer<State, API= any> {
    getState() : State;
    getAPI() : API;
}

class MyAPI<State> {
    state: State;
    my() {}
    api() {}
    methods() {}
    here() {}
}

class MySerializer<State> implements Serializer<State, MyAPI<State>> {
    api = new MyAPI<State>();

    getState() {
        return this.api.state;
    }

    getAPI() {
        return this.api;
    }
}

class AllowCustomSerializer<T= any, S extends Serializer<T> = MySerializer<T>> {
    serializer: S;

    get api() {
        return this.serializer.getAPI();
    }

}

interface MyState {}
const instance = new AllowCustomSerializer<MyState, MySerializer<MyState>>();
instance.serializer.getAPI() // type of `API`
instance.api // type of `any` 😢