asyncReduce.js

var sum = 0;

var add5Async = () => { 
  return setTimeoutAsync(1000)
    .then(add5.bind(null, sum)) // pure function, bind global variable as input
    .then(setSum); // side-effect at the end
};

asyncReduce([ add5Async, add5Async, add5Async ])
  .then(() => { console.log('sum:', sum); })
  .catch((e) => { console.log(e); } );

function add5(number) { return number + 5; }
function setSum (_) { sum = _; }

function setTimeoutAsync(delay) {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      try { resolve(); }
      catch (e) { reject(e); }
    }, delay);
  });
}

function asyncReduce(promises) {
  return promises.reduce((chain, fn) => chain.then(fn), Promise.resolve());
}