Iterations with asynchronous functions

Question

I haven't really worked much with asynchronous code so I'm wondering if I am doing something wrong/something could go wrong. Right now of all the test cases that I have done, my code has worked but I'm not sure if I will get side effects or not.

I won't post any of the business logic, just the iteration part with the asynchronous function in it:

function asyncInIteration(skinArray)  {
   request("/someaddresshere", function (err, response, data)  {

      var left = something.length;

      for (var i = 0;i<something.length;i++) (function (i) {
          request("/someotheraddress", function (err, response, data) {
             if (--left === 0)  {
                 console.log("End of for loop");
             }
          })
      })(i);
   });
}

Basically, what I am trying to do with this code is loop through an array (skinArray) and add values to it via sending HTTP requests.

The most important thing for me is that:

1. Every value of i should only be repeated once.
2. When I call --left === 0, that has to be called once, and only after all values of i have been gone through.

If someone wants to see the code with the business logic, you can find it here.

Answer 1

Use lodash to make Your iteration "readable".

And use async to handle end of asynchronous iteration to properly respond with "end of loop"

var _ = require('lodash');

var something = ['a','b','c'];
function asyncInIteration(skinArray)  {

   request("/someaddresshere", function (err, response, data)  {
      var functions = []; // array of functions that will be called

      // generating array of functions
      _.times(something.length, 
          function() {
              // pushing function to functions list
              functions.push(function(done){ // "next" is necessary to inform async module about finishing call
                  request("/someotheraddress", function (err, response, data) {
                      // doing something
                      console.log("\n--------");
                      console.log("RESPONSE:", data);
                      console.log("--------\n");
                      done();
                  });
              });
          });

      // can change to .series if You want to make it synchronous (not parallel)
      async.parallel(functions, function(){
          console.log('End of loop');
      });
   });
}

P.S. In fact my code looks like not so simple, but there is no another elegant way to control asynchronousity. Maybe Promise object will be easy to work, but I don't think it's suitable for current task.