decorator.ts

export default null // モジュールモードを強制します

type DebuggableClassConstructor = {
  new(...args: any[]): {getDebugValue(): object}
}

function debuggable<
  C extends DebuggableClassConstructor
>(Class: C) {
  return class extends Class {
    debug() {
      console.log(this.getDebugValue())
    }
  }
}

class User {
  constructor(
    private name: string
  ) {}

  getDebugValue() {
    return {
      name: this.name
    }
  }
}

/*
let user = new User('fukata')
user.debug()
// decorator.ts:31:6 - error TS2339: Property 'debug' does not exist on type 'User'.
*/

let user = new (debuggable(User))('fukata')
user.debug()
// $./node_modules/.bin/ts-node decorator.ts
// { name: 'fukata' }