javascript - 循环内的 Node.js 回调具有错误的迭代器值

Question

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JavaScript closure inside loops – simple practical example (45 个回答)

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for(var i in companyTickerList) {
  console.log('i = ' + i);
  //construct url
  var url = base_url + companyTickerList[i];
  console.log('url = ' + url);
  request(url, function(error, response, xml) {
    if(!error && response.statusCode == 200) {
      //load xml returned from GET to url
      var cik_xml = cheerio.load(xml)
      console.log('i = ' + i);
      //map company ticker symbol to cik value scraped from xml
      TickerToCIK[companyTickerList[i]] = cik_xml('company-info cik').text();
      console.log('TICKER = ' + companyTickerList[i] + ' CIK = ' + cik_xml('company-info cik').text());
    }  
          
}

//CONSOLE LOG OUTPUT
i = 0
http://www.sec.gov/cgi-bin/browse..........SNPS
i = 1
http://www.sec.gov/cgi-bin/browse..........IBM
i = 2
http://www.sec.gov/cgi-bin/browse..........BA
i = 3
http://www.sec.gov/cgi-bin/browse..........GM
i = 4
http://www.sec.gov/cgi-bin/browse..........F
i = 5
http://www.sec.gov/cgi-bin/browse..........C
i = 6
http://www.sec.gov/cgi-bin/browse..........CVX

i = 6
TICKER = CVX CIK = 0000883241
i = 6
TICKER = CVX CIK = 0000037996
i = 6
TICKER = CVX CIK = 0000831001
i = 6
TICKER = CVX CIK = 0000093410
i = 6
TICKER = CVX CIK = 0001467858
i = 6
TICKER = CVX CIK = 0000012927
i = 6
TICKER = CVX CIK = 0000051143

为什么每次请求调用触发后，回调函数中的迭代器值 i 总是等于 6？它使我的 TickerToCIK map 的键始终为 CVX。我需要将 i 作为参数传递给回调函数吗？

Answer 1

var 很棘手:)JavaScript 实际上是这样评估你的代码的:

var i;
for(i in companyTickerList) {
  console.log('i = ' + i);
  // ...

意思是，它就像在第一行代码之前执行的 var 定义。

因此，您实际上有一个变量 i，它会更新 6 次，最终 i = 6。

您的 request() 回调是异步，当您的第一个回调实际被调用时，循环早已消失，i等于 6。

解决方案:

一种可能的解决方案是使用IIFE(立即调用函数表达式)。

即:

    (function (i) {
        // async code using i 
    })(i);

像这样:

for (var i in companyTickerList) {
    console.log('i = ' + i);
    //construct url
    var url = base_url + companyTickerList[i];
    console.log('url = ' + url);

    (function (i) {

        request(url, function (error, response, xml) {
            if (!error && response.statusCode == 200) {
                //load xml returned from GET to url
                var cik_xml = cheerio.load(xml);
                console.log('i = ' + i);
                //map company ticker symbol to cik value scraped from xml
                TickerToCIK[companyTickerList[i]] = cik_xml('company-info cik').text();
                console.log('TICKER = ' + companyTickerList[i] + ' CIK = ' + cik_xml('company-info cik').text());
            }
        });

    })(i);
}