async.js

var eachLimit = function (arr, limit, iterator, callback) {
  callback = callback || function () {};
  if (!arr.length || limit <= 0) {
    return callback();
  }

  var completed = 0;
  var started = 0;
  var running = 0;

  (function replenish() {
    if (completed >= arr.length) {
      return callback();
    }

    while (running < limit && started < arr.length) {
      started += 1;
      running += 1;
      iterator(arr[started - 1], function (err) {
        if (err) {
          callback(err);
          callback = function () {};
        } else {
          completed += 1;
          running -= 1;
          if (completed >= arr.length) {
            callback();
          } else {
            replenish();
          }
        }
      });
    }
  })();
};

var retry = function (times, iterator, callback) {
  var next = function (index) {
    iterator(function (err, data) {
      if (err && index < times) {
        next(index + 1);
      } else {
        callback(err, data);
      }
    });
  };
  if (times < 1) {
    callback();
  } else {
    next(1);
  }
};

var async = {
  eachLimit: eachLimit,
  retry: retry,
};

module.exports = async;