Node.js callback style; run multiple async tasks in parallel

asyncParallel.js

var asyncParallel = function(tasks, callback) {
  var results = [];
  var count = tasks.length;
  tasks.forEach(function(task, index) {
    task(function(err, data) {
      results[index] = data;
      if (err) {
        callback && callback(err);
        callback = null;
      }
      if (--count === 0 && callback) {
        callback(null, results);
      }
    });
  });
};

usage.js

asyncParallel([
  function(cb) {
    setTimeout(function() {
      cb(null, 'one');
    }, 2000);
  },
  function(cb) {
    setTimeout(function() {
      cb(null, 'two');
    }, 1000);
  }
], function(err, results) {
  if (err) return console.log(err);
  console.log(results);
});