TypeScriptで匿名クラス・内部クラス ref: https://qiita.com/kurogelee/items/fd7b86ba5d5660180bb3

anon.ts

class Base {
    public static create() {
        return new class extends Base {
            constructor() {
                super("XXX");
            }
        }();
    }

    constructor(private value: string) {
    }
    public methodA() {
        console.log("A" + this.value);
    }
}

const v = Base.create();
v.methodA();

file1.txt

class Base {
    public static Inner = class extends Base {
        public method() {
            return 1;
        }
    };
}

inner.ts

class Base {
    public static Inner = class extends Base {
        public method() {
            return 1;
        }
    };
}