chain.js

function chain(prev = null) {
  const cur = () => {
    if (cur.prev) {
      cur.prev.next = cur;
      cur.prev();
    } else {
      cur.forward();
    }
  };

  cur.prev = prev;
  cur.fn = null;
  cur.args = null;

  cur.do = (fn, ...args) => {
    cur.fn = fn;
    cur.args = args;
    return chain(cur); 
  };

  cur.forward = () => {
    if (cur.fn) cur.fn(cur.args, () => {
      if (cur.next) cur.next.forward(); 
    });
  };

  return cur;
}