Async mapping.

async_map.js

var asyncMap = function(tasks, finalCb){
  var results = [];
  var completed = 0;
  var checkCompleteness = function() {
    if (completed === tasks.length) {
      finalCb(results);
    }
  };
  var runTask = function (i) {
    var onResult = function(val) {
      results[i] = val;
      completed++;
      checkCompleteness();
    };
    tasks[i](onResult);
  }
  for(var i = 0; i < tasks.length; i++) {
    // make `i` available in the closure scope
    // so it can be accessed by `onResult`
    runTask(i); 
  }
};