如何正确处理 Promise.all：未定义

Posted 2023-03-08

【中文标题】如何正确处理 Promise.all：未定义

【英文标题】：How to handle Promise.all properly: Getting undefined

【发布时间】：2018-05-23 18:11:22

【问题描述】：

我正在尝试从向不同 REST API 发出的一些请求中获取一个填充了信息的数组。 我曾想过使用Promise.all 来做到这一点，但由于某种原因，它会产生一个内部包含一堆undefined 的数组。

[未定义，未定义，未定义，未定义]

这是我的代码：

var _ = require("lodash");//Used exclusively to check if the result from the request is an object
var ccxt = require("ccxt");//External library used to make the requests

let pairs = ["ETH/EUR", "BTC/EUR", "LTC/EUR", "BCH/EUR"]; //Array on which the Promise.all is based

function test(p) 
  for (var i = 0; i < ccxt.exchanges.length; i++)  //Looping through all the rest APIs
    let exchange = new ccxt[ccxt.exchanges[i]](); //Defining each API to make the requests
    if (exchange.hasFetchOrderBook) 
      exchange //Beginning of the request
        .fetchOrderBook(p)
        .then(order => 
          if (_.isObject(order) && order.bids[0][1]) 
            let now = Math.floor(new Date());
            order.mkt = exchange.name;
            order.pair = p;
            order.ping = now - order.timestamp;
            return order; //Return the result of the request
          
        )
        .catch(e => );
    
  


Promise.all(pairs.map(test)) //Making the requests based on the Pairs Array
  .then(res => 
    console.log(res); //Logging the results ==> [undefined, undefined, undefined, undefined] for some reason...
  )
  .catch(e => 
    console.log(e);
  );

我知道请求是正确发出的，因为如果我在循环中 console.log 记录 order，我会得到正确的结果——记录时的结果示例：

 bids: 
   [ [ 12009.52, 0.0468 ],
     [ 12008.5, 0.0227 ],
     [ 12007.48, 30.9321 ],
     [ 12006.46, 0.0537 ],
     [ 12005.45, 0.0157 ],
     [ 12004.43, 7.1659 ],
     [ 12003.41, 0.0164 ],
     [ 12002.39, 23.4159 ],
     [ 12001.38, 0.0284 ],
     [ 12000.36, 0.0132 ],
     [ 11999.34, 0.0194 ],
     [ 11998.33, 0.0034 ],
     [ 11997.31, 7.526 ],
     [ 2445.72, 34.075 ],
     [ 2445.17, 25.4842 ],
     [ 2444.96, 0.1118 ],
     [ 2444.75, 23.288 ],
     [ 2444, 0.0247 ],
     [ 2443.8, 0.192 ],
     [ 765.51, 0.0828 ] ],
  asks: 
   [ [ 12048.74, 2.523 ],
     [ 12049.77, 0.0159 ],
     [ 12050.79, 0.029 ],
     [ 12051.82, 0.0061 ],
     [ 12052.84, 0.0181 ],
     [ 12053.87, 0.0164 ],
     [ 12054.89, 0.0355 ],
     [ 12055.92, 0.0042 ],
     [ 13419.62, 0.0063 ],
     [ 13420.64, 0.0174 ],
     [ 13421.78, 0.0143 ],
     [ 13422.92, 0.026 ],
     [ 13424.06, 0.0055 ],
     [ 13425.2, 14.4552 ],
     [ 13426.23, 0.0065 ],
     [ 13427.25, 0.0057 ],
     [ 13428.39, 0.0147 ],
     [ 13429.53, 4.0375 ],
     [ 13430.56, 23.9541 ],
     [ 13431.58, 0.0137 ] ],
  timestamp: 1512845715447,
  datetime: '2017-12-09T18:55:15.447Z',
  mkt: 'LakeBTC',
  pair: 'BTC/EUR',
  ping: 0 

所以我想我正在处理的问题与函数的异步特性有关......但我不确定如何让它同步。 再次，只是为了澄清我的问题：目标是获得一个包含 4 种不同类型对象的数组（每对一个 --> 数组），以便我可以对每个对象进行操作。 为了更清楚起见，这里是我要实现的目标的说明：

[
  [
    Object1,
    Object2,
    Object3,
    etc...
  ],
  [
    Object1,
    Object2,
    Object3,
    etc...
  ],
  [
    Object1,
    Object2,
    Object3,
    etc...
  ],
  [
    Object1,
    Object2,
    Object3,
    etc...
  ]
]

为什么Promise.all 不等待请求结果就返回数组？

我希望这已经足够清楚了！如果不让mw知道！ :P

提前感谢您的帮助！

【问题讨论】：

您的函数 test 不会返回任何内容，因此 undefined。

嘿@elclanrs！它不返回订单吗？ --> return order; //Return the result of the request

那个return是inside另一个函数，所以它返回到那个函数，而不是外部那个。

【参考方案1】：

您的test 函数确实返回undefined。您需要为结果返回一个承诺：

function test(p) 
  return Promise.all(ccxt.exchanges.map(api =>  //Looping through all the rest APIs
//^^^^^^^^^^^^^^^^^^
    let exchange = new ccxt[api](); //Defining each API to make the requests
    if (exchange.hasFetchOrderBook) 
      return exchange //Beginning of the request
        .fetchOrderBook(p)
        .then(order => 
          if (_.isObject(order) && order.bids[0][1]) 
            let now = Math.floor(new Date());
            order.mkt = exchange.name;
            order.pair = p;
            order.ping = now - order.timestamp;
            return order; //Return the result of the request
          
          // else undefined
        )
        .catch(e => ); // undefined
    
    // else undefined
  ));

当然，当if 条件不适用或发生错误时，您的承诺仍会通过undefined 实现。