A partial Ecmascript promise implementation written in typescript

Promise2.ts

type InnerPromise2State<T> =
  | {
      type: "idle";
    }
  | { type: "resolved"; value: T }
  | { type: "rejected"; error: unknown };

type Consumer<T> = (value: T) => void;

type Promise2Executor<T> = (
  resolve: Consumer<T>,
  reject: Consumer<unknown>,
) => unknown;

type CatchCallback = (value: any) => unknown;
type OkCallback<T> = (value: T) => unknown;

export class Promise2<T> {
  #state: InnerPromise2State<T> = { type: "idle" };

  #listenersOk: OkCallback<T>[] = [];
  #listenersErr: CatchCallback[] = [];

  constructor(task: Promise2Executor<T>) {
    try {
      task(this.#resolve.bind(this), this.#reject.bind(this));
    } catch (err) {
      this.#reject(err);
    }
  }

  #resolve(value: T): void {
    if (this.#state.type === "idle") {
      this.#state = { type: "resolved", value };
      this.#verifyState();
    }
  }
  #reject(error: unknown): void {
    if (this.#state.type === "idle") {
      this.#state = { type: "rejected", error };
      this.#verifyState();
    }
  }

  #verifyState() {
    switch (this.#state.type) {
      case "idle":
        break;
      case "resolved": {
        this.#listenersOk.forEach((listener) =>
          queueMicrotask(() =>
            listener((this.#state as { type: "resolved"; value: T }).value),
          ),
        );
        break;
      }
      case "rejected": {
        this.#listenersErr.forEach((listener) =>
          queueMicrotask(() =>
            listener(
              (this.#state as { type: "rejected"; error: unknown }).error,
            ),
          ),
        );
        break;
      }
    }

    if (this.#state.type !== "idle") {
      this.#listenersErr.splice(0, this.#listenersErr.length);
      this.#listenersOk.splice(0, this.#listenersOk.length);
    }
  }

  then(okCb: OkCallback<T>, errCb?: ErrorCallback) {
    this.#listenersOk.push(okCb);
    if (errCb) {
      this.#listenersErr.push(errCb);
    }
    this.#verifyState();
  }

  catch(cb: ErrorCallback) {
    this.#listenersErr.push(cb);
    this.#verifyState();
  }
}