Promise 必知必会（十道题）

来源：https://juejin.im/post/5a04066351882517c416715d

设计并实现 Promise.race() 链接

const _race = (p)=>{
    return new Promise((resolve, reject)=>{
        p.forEach((item)=>{
            Promise.resolve(item).then(resolve, reject)
        })
    })
}

模拟实现一个 Promise.finally 链接

Promise.prototype.finally = function (callback) {
  let P = this.constructor;
  return this.then(
    value  => P.resolve(callback()).then(() => value),
    reason => P.resolve(callback()).then(() => { throw reason })
  );
};