How to structure nested Promises

Question

I have a situation where I think the only choice for me is to nest some Promises within each other. I have a Promise that needs to be performed and a method that does someth

Answer 1

Use .then()

let doStuff = (resolve, reject) => {/* resolve() or reject() */};
let promise = new Promise(doStuff);
doSomethingUntilPromiseisDone(
  promise 
  .then(value => fetchValue(url))
  .then(value => value.blob())
  .then(saveToCache)
)
.then(success => console.log("success!!"))
.catch(err => console.error(err))

Answer 2

Each function will call the next one with the result of the method before.

var promises = [1,2,3].map((guid)=>{
    return (param)=> {
      console.log("param", param);
      var id = guid;
      return new Promise(resolve => {
        // resolve in a random amount of time
        setTimeout(function () {
          resolve(id);
        }, (Math.random() * 1.5 | 0) * 1000);
      });
    }
}).reduce(function (acc, curr, index) {
  return acc.then(function (res) {
    return curr(res[index-1]).then(function (result) {
      console.log("result", result);
      res.push(result);
      return res;
    });
  });
}, Promise.resolve([]));
promises.then(console.log);

Answer 3

you can use generator to flatten your nested promises (Bluebird.couroutine or Generators)

//Bluebird.couroutine
const generator = Promise.coroutine(function*() {
  try {
     const value = yield fetchValue(url);
     const success = yield saveToCache(value);
     console.log('success:', success);
  } catch(e) {
     console.error(err);
  }    
}));

generator();