Promise Collection Waterfall

PromiseCollectionWaterfall.js

const collection = [{id:'A'},{id:'B'},{id:'C'},{id:'D'}];

function doStuff(val) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(val);
    }, 1000);
  });
}


var promises = collection.reduce((acc, curr) => {
  
  return acc.then((res) => {
    
    return doStuff(curr).then(result => {
      console.log('> id:'+ result.id + ' '+ new Date().getSeconds());
      res.push(result);
      return res;
    });
  });
  
}, Promise.resolve([]));

promises.then(console.log);

// Jsbin: https://jsbin.com/wifonol/3/edit?js,console
// Example OUTPUT:
// > id:A 46
// > id:B 47
// > id:C 48
// > id:D 49