Promise 的一些扩展方法

// promise.done()
Promise.prototype.done = function (onFulfilled, onRejected) {
  this.then(onFulfilled, onRejected)
    .catch(function (reason) {
      // 抛出一个全局错误
      setTimeout(() => { throw reason }, 0)
    })
}

// promise.finally()
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 })
  )
}

// promise.if()
Promise.prototype.if = function(flag, planAS, planBS) {
    return flag
		? Promise.all(planAS.map(f => f()))
		: Promise.all(planBS.map(f => f()))
}