class MyPromise {
  constructor(executor) {
    if (typeof executor !== 'function') {
      throw new Error('Executor function must be a function');
    }

    this.$state = 'PENDING';
    this.$chained = [];

    const resolve = (res) => {
      if (this.$state !== 'PENDING') {
        return;
      }
      this.$state = 'FULFILLED';
      this.$internalValue = res;
      for (const { onFulfilled } of this.$chained) {
        onFulfilled(res);
      }
    };
    const reject = (err) => {
      if (this.$state !== 'PENDING') {
      }
      this.$state = 'REJECTED';
      this.$internalValue = err;
      for (const { onRejected } of this.$chained) {
        onRejected(err);
      }
    };

    try {
      executor(resolve, reject);
    } catch (err) {
      reject(err);
    }
  }

  then(onFulfilled, onRejected) {
    if (this.$state === 'FULFILLED') {
      onFulfilled(this.$internalValue);
    } else if (this.$state === 'REJECTED') {
      onRejected(this.$internalValue);
    } else {
      this.$chained.push({ onFulfilled, onRejected });
    }
  }
}

const firstPromise = new MyPromise((resolve, reject) => {
  setTimeout(() => resolve('wubadubdub'), 1000);
});

firstPromise.then(successMessage => console.log(`wahoo ${successMessage}`));