JavaScript Promise implementation

promise.js

function addHandler(fn, arg, resolve, reject) {
  try {
    const result = fn(arg);
    if (result instanceof MyPromise) {
      result.then(resolve).catch(reject);
    } else {
      process.nextTick(() => resolve(result));
    }
  } catch (err) {
    process.nextTick(() => reject(err));
  }
}

class MyPromise {
  #status = "started";
  #thenFn = null;
  #catchFn = null;

  static resolve(arg) {
    return new MyPromise((resolve) => {
      process.nextTick(() => resolve(arg));
    });
  }

  static reject(err) {
    return new MyPromise((_resolve, reject) => {
      process.nextTick(() => reject(err));
    });
  }

  constructor(fn) {
    fn(this.#resolve, this.#reject);
  }

  #resolve = (arg) => {
    if (this.#status === "started") {
      this.#status = "resolved";
      this.#thenFn?.(arg);
    }
  };

  #reject = (arg) => {
    if (this.#status === "started") {
      this.#status = "rejected";
      if (!this.#catchFn) {
        console.warn("Unhandled promise rejection");
      }
      this.#catchFn?.(arg);
    }
  };

  then(fn) {
    return new MyPromise((resolve, reject) => {
      this.#thenFn = (arg) => {
        addHandler(fn, arg, resolve, reject);
      };
      this.#catchFn = reject;
    });
  }

  catch(fn) {
    return new MyPromise((resolve, reject) => {
      this.#catchFn = (arg) => {
        addHandler(fn, arg, resolve, reject);
      };
      this.#thenFn = resolve;
    });
  }
}

// Tests

new MyPromise((resolve) => setTimeout(() => resolve(), 1000))
  .then(() => {
    console.log(1);
  })
  .then(() => {
    console.log(2);
    throw new Error("Error 3");
  })
  .catch((err) => {
    console.log(3);
    console.log(err);
  })
  .then(() => {
    console.log(4);
    return MyPromise.resolve(5);
  })
  .then((arg) => {
    console.log(arg);
    return new MyPromise((resolve) => setTimeout(() => resolve(6), 100));
  })
  .then((arg) => {
    console.log(arg);
    return new MyPromise((_resolve, reject) => setTimeout(() => reject(7), 100));
  })
  .catch((err) => {
    console.log(err);
    return MyPromise.reject(8);
  })
  .catch((err) => {
    console.log(err);
  })
  .then(() => {
    console.log("fin");
  });