Async Map  -  A Simple Implementation Without Promises

asyncMap.js

// asyncMap takes two arguments, a collection of tasks and a callback to run on those tasks
var asyncMap = function (tasks, cb) {
  // declare a counter - set it at zero
  var counter = 0;
  // here we are given tasks as an object
  // it's possible tasks is an array
  // get number of tasks we need to come back
  var numberTask = Object.keys(tasks).length;
  // object we will run the final cb on when all tasks have returned
  var results = {};
  
  // loop through tasks
  for (var key in tasks) {
    // why are we wrapping an anonymous function here?
    // we don't know when tasks come back
    // the key may not be what was originally intended
    // creating a function allows us to maintain scope
    // this guarantees we pass in the 'key' we want
    (function (key) {    
      // purposely not considering what happens if error returned
      // we are defining the callback passed to each task
      // tasks[key]() is an invocation of a task that has a predetermined callback
      // we are intercepting the result of that callback and something something more with it
      tasks[key](function(res) {
        // store the result of task in the results storage
        results[key] = res;
        // increment our counter
        counter++
        // check if we got all our tasks to comeback
        // if so then run our cb passed in to asyncMap
        if (counter === numberTask) {
          cb(results);
        }
      })
    // invoke our anonymous function with key to create scope
    })(key)
  }
}