class Promises {
  constructor(fn) {
    this.state = 0;
    this.value = undefined;
    this.todos = [];
    fn(() => this.resolve(), () => this.reject());
  }
  resolve(value) {
    if (this.state !== 0) return;
    this.state = 1;
    this.value = value;
    this.run();
  }
  reject(reason) {
    if (this.state !== 0) return;
    this.state = 2;
    this.value = reason;
    this.run();
  }
  run() {
    let callbackName, resolver;
    if (this.state === 0) return;
    if (this.state === 1) {
      callbackName = 'onFulfilled';
      resolver = 'resolve';
    }
    if (this.state === 2) {
      callbackName = 'onRejected';
      resolver = 'reject';
    }
    setTimeout(() => {
      this.todos.forEach(todo => {
        try {
          let cb = todo[callbackName];
          if (cb) todo.resolve(cb(this.value));
          else todo[resolver](this.value);
        } catch (e) {
          todo.reject(e);
        }
        this.todos.shift();
      });
    });
  }
  then(onFulfilled, onRejected) {
    let todo = new Promises(() => {});
    todo.onFulfilled =
      typeof onFulfilled === 'function' ? onFulfilled : null;
    todo.onRejected = typeof onRejected === 'function' ? onRejected : null;
    this.todos.push(todo);
    this.run();
    return todo;
  }
}