实现Promise.finally()

// 方法1
Promise.prototype.finally = function (cb) {
  let P = this.constructor
  return this.then(
    value => P.resolve(cb()).then(() => value),
    reason => P.resolve(cb()).then(() => { throw reason })
  )
}


// 方法2
window.Promise && !('finally' in Promise) && !function() {        
  Promise.prototype.finally = function(cb) {
    cb = typeof cb === 'function' ? cb : function() {}
      
    // 获取当前实例构造函数的引用
    let Fn = this.constructor  

    // 接受状态：返回数据
    let onFulfilled = function(data) {
      return Fn.resolve(cb()).then(function() {
        return data
      })
    }

    // 拒绝状态：抛出错误
    let onRejected = function(err) {
      return Fn.resolve(cb()).then(function() {
        throw err
      })
    }

    return this.then(onFulfilled, onRejected)
  }
}()