javascript - 在循环中构建数组后发送后无法设置 header

Question

我正在尝试构建一个对象数组并发送给客户端。接收错误:

    throw new Error('Can\'t set headers after they are sent.');
    ^

Error: Can't set headers after they are sent.
    at ServerResponse.OutgoingMessage.setHeader (_http_outgoing.js:357:11)
    at ServerResponse.header (D:\Dropbox (Personal)\coding\coinTracker5\node_modules\expr                         ess\lib\response.js:725:10)
    at ServerResponse.send (D:\Dropbox (Personal)\coding\coinTracker5\node_modules\expres                         s\lib\response.js:170:12)
    at ServerResponse.json (D:\Dropbox (Personal)\coding\coinTracker5\node_modules\expres                         s\lib\response.js:256:15)
    at D:\Dropbox (Personal)\coding\coinTracker5\routes\index.js:58:11
    at D:\Dropbox (Personal)\coding\coinTracker5\node_modules\cryptox\lib\index.js:51:9
    at D:\Dropbox (Personal)\coding\coinTracker5\node_modules\cryptox\lib\bitfinex.js:93:                         13
    at D:\Dropbox (Personal)\coding\coinTracker5\node_modules\cryptox\lib\bitfinex.js:72:                         13
    at Request._callback (D:\Dropbox (Personal)\coding\coinTracker5\node_modules\bitfinex                         -api-node\rest.js:95:16)
    at Request.self.callback (D:\Dropbox (Personal)\coding\coinTracker5\node_modules\requ                         est\request.js:187:22)

我认为 res.json(rates) 仅触发一次，所以我不确定为什么 header 被设置两次？

如果有更好的方法来构建这个数组并将其发送给客户端，请赐教!谢谢

var rates = [];
for (var i = 0, len = cryptoxArray.length; i < len; i++) {

  cryptoxArray[i].getRate({
    pair: "XBT_USD"
  }, function(err, rateResponse) {
    if (!err)
      rates.push(rateResponse);

    if (i == len) {
      console.log('rates: ' + rates);
      res.json(rates);
    }
  });

Answer 1

当任何回调被调用时，我已经等于长度，你不能使用这样的循环。我的建议是使用 promise 并为此或 iife 执行 Promose.all。

生活

for(i, len, etc){
  (function(index){do your stuff here})(i)
}

我个人最喜欢的就是类似的东西。

import Promise from 'bluebird';

Promise.all(
 cryptoxArray.map(
   item => Promise.promisify(item. getRate)({pair: "XBT_USD"})
   .then(rate => rate, error => null)
 ) 
)
.then(rates => rates.filter(rate => rate !== null))
.then(res.json);