NodeJS可变碰撞？ 与请求异步/同步

Question

Issue : Data INFO_X sometimes and randomly become Null.

Question ,

Does the variable overwrites each other for INFO_1 INFO_2 INFO_3 since Nodejs run fast unlike PHP it follows a sequence/step by step.

I checked for NULLS before doing a request but the debug shows it's NOT NULL before executing the 2nd request, at random, random variables will become null upon submitting the 2nd request.

I also checked my source is definitely not returning any null.

Is the variable being overwritten before the 2nd request is sent or what? Please advice.

var request = require('request');

var urls = [ 'URL',
             'URL',
             'URL'];

urls.forEach(processUrl);

function processUrl(url) {

  request(url, function (error, response, body) {
  if (!error) {
    var obj = JSON.parse(body);
     for (var i = 0, len = obj['products'].length; i < len; ++i) {
         var data = obj['products'][i];
         var INFO_1 = data.INFO_1
         var INFO_2 = data.INFO_2
         var INFO_3 = data.INFO_3

        request("URL/POSTINFO?INFO_1="+INFO_1+"&INFO_2="+INFO_2+"&INFO_3="+INFO_3+"&seller_id=", function(error, response, body) {
            console.log(body);
        });

     }
  }
});

}

Answer 1

Yes, that is the case, because request function is asynchronous. I wouldn't call nodejs "faster" than PHP, it just runs asynchronous request methods, while PHP is generally synchronous.

You could resolve the issue with promises, eg Promise.all([]) and provide an array of request functions (see here: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Promise/all ), or in your case, use this library https://github.com/caolan/async#forEach

Since you're using callbacks in the request function, your best option is to use async as provided in the link above. Here's an example code:

    function request(url, cb) {
      setTimeout(() => {
        cb(url + ' accessed at ' + new Date());
      }, 2000);

    }

    var urls = ['URL1', 'URL2', 'URL3'];

    async.each(urls, (item) => {
      console.log(item);
      request(item, (value) => {
        request(value, (newValue)=>{
          console.log(newValue);
        });
      });
    }, (err) => {
      console.log(err);
    });

Here's a working example: https://plnkr.co/edit/Q5RAvKdaLxV9cUT4GP4w?p=preview