How to return from a looped asynchronous function with callback in Node

I am trying to write a function that:

1. Takes an array of URLs
2. Gets files from URLs in parallel (order's irrelevant)
3. Processes each file
4. Returns an object with the processed files

Furthermore, I don't need for errors in #2 or #3 to affect the rest of the execution in my application in any way - the app could continue even if all the requests or processing failed.

I know how to fire all the requests in a loop, then once I have all the data, fire the callback to process the files, by using this insertCollection pattern.

However, this is not efficient, as I shouldn't need to wait for ALL files to download before attempting to process them - I would like to process them as each download finishes.

So far I have this code:

const request = require('request');

const urlArray = [urlA, urlB, urlC];
const results = {};

let count = 0;
let processedResult;  

const makeRequests = function (urls, callback) {
    for (let url of urls) {
      request(url, function(error, response, body) {
        if (error) {
          callback(error);
          return;
        }
        processedResult = callback(null, body)
        if (processedResult) {
          console.log(processedResult); // prints correctly!
          return processedResult;
        }
      })
    }
};

const processResult = function(error, file) {
    if (error) {
      console.log(error);
      results.errors.push(error);
    }

    const processedFile = file + `<!-- Hello, Dolly! ${count}-->`;
    results.processedFiles.push(processedFile);

    if (++count === urlArray.length) {
      return results;
    }
  };

const finalResult = makeRequests(urlArray, processResult);
console.log(finalResult); // undefined;

In the last call to processResult I manage to send a return, and makeRequests captures it, but I'm failing to "reign it in" in finalResult after that.

My questions are:

1. Why is this not working? I can print a well-formed processedResult on the last iteration of makeRequests, but somehow I cannot return it back to the caller (finalResult)
2. How can this be solved, ideally "by hand", without promises or the help of libraries like async?

via alexcs