🐥 Example of type solution using d.ts in JavaScript.

index.js

/** @type {import("./Piyo").default} */
let x;
let y = new x();

Piyo.d.ts

export interface PiyoInstance {
  /**
   * Thru an argument.
   * @param x argument.
   * @template T argument type.
   */
  instanceMethod<T>(x: T): T;
}

/** 🐥 */
export default interface Piyo {
  /** Create the instance. */
  new (): PiyoInstance;
  /**
   * Thru an argument.
   * @param x argument.
   * @template T argument type.
   */
  staticMethod<T>(x: T): T;
}