async.js

// initial: value to call first task with
// tasks: varargs, functions to be called in order
// each task must take a callback function and optionally the result of the previous task
// the task must either call callback (with a result) or throw an error.
function async(initial, tasks) {
  if (arguments.length === 0) throw new Error('argument required');
  var chain = Array.from(arguments).slice(1);
  var head = -1;
  var top = chain.length - 1;

  callback(initial);

  function callback(result) {
    head++;
    if (head > top) return;
    var task = chain[head];
    task(callback, result);
  }
}