mutexify

var mutexify = function (fn, lim) {

  var queue = [];
  var inProgress = 0;

  var invokeNext = function () {
    if (queue.length === 0) {
      return;
    }
    if (inProgress >= lim) {
      return;
    }

    inProgress += 1;

    var next = queue.shift();

    fn(next.arg, function () {
      next.cb.apply(fn, arguments);
      inProgress -= 1;

      setTimeout(invokeNext, 0);
    });
  };

  return function (arg, cb) {

    queue.push({
      arg: arg,
      cb: cb
    });

    setTimeout(invokeNext, 0);
  };
};