//  First approach: pass functions as arguments for chaining. Executes from left to right;
//  Please, run with node >= 8 (because of asserts)

function chain() {
  let funcs = [...arguments];
  let finish;

  function run(acc) {
    if (funcs.length > 0) {
      const func = funcs.shift();
      if (typeof func === 'function') {
        func(acc, (err, res) => {
          if (err) {
            finish(err);
          } else {
            run.call(null, res);
          }
        });
      }
    } else {
      finish(null, acc);
    }
  }

  return function (number, cb) {
    finish = cb;

    run(number);
  }
}

var shouldBeTwelve = chain(plusTwo, timesTwo, plusTwo);

shouldBeTwelve(3, (err, result) => {
  assert.equal(result, 12)
});