Serial promises

serialPromises.js

const keyedObj = {
  foo: 'fooVal',
  bar: 'barVal',
  baz: 'bazVal'
};

function concatAsync(acc, val) {
  console.log(`processing '${val}'`);
  return new Promise(resolve => {
    resolve(acc.concat(val));
  });
}

function concatKeysAsync(obj) {
  let p = Promise.resolve([]);
  
  Object.keys(obj).forEach(k => {
    p = p.then(res => {
      return concatAsync(res, k);
    }); 
  });
  
  return p;
}

concatKeysAsync(keyedObj).then(res => {
  console.log('all done', res);
});

/*

$ babel-node serialPromises.js
processing 'foo'
processing 'bar'
processing 'baz'
all done [ 'foo', 'bar', 'baz' ]

*/