手写promise的then方法

// 原生实现promise基本功能，实现依次调用，eg：a().then()
//不用考虑实现race all finally的复杂功能
function Promise(fn) {
  if (typeof fn !== 'function') {
    throw new Error('Resolver is not a Function')
  } else {
    this.callbacks = [];
    const resolve = val => {
      setTimeout(() => {
        this.callbacks.forEach(cb => {
          cb(val)
        });
      }, 0)
    };
    fn(resolve);
  }
}
Promise.prototype.then = function (newCallBack) {
  this.callbacks.push(newCallBack);
};
const a = new Promise(function (resolve) {
  setTimeout(() => {
    resolve('执行函数参数')
  }, 2000)
})
a.then(function (val) {
  console.log(val);
});