javascript - How to structure nested Promises - Stack Overflow

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 something until that Promise is plete. Something like this:

let promise = new Promise((resolve, reject) => {

  // Do some stuff

});

doSomethingUntilPromiseisDone(promise);

However, within my Promise, I need to execute another method that returns another Promise:

let promise = new Promise((resolve, reject) => {

  fetchValue(url)
    .then((value) => {

      // Do something here

    }).catch((err) => {
      console.error(err);
    });

});

doSomethingUntilPromiseisDone(promise);

But now, in the fetchValue method's then statement, I have another method I need to execute that, guess what, returns another Promise:

let promise = new Promise((resolve, reject) => {

  fetchValue(url)
    .then((value) => {

      saveToCache(value)
        .then((success) => {

          console.log('success!!');
          resolve('success');

        });

    }).catch((err) => {
      console.error(err);
    });

});

doSomethingUntilPromiseisDone(promise);

So in the end, I have a Promise, within a Promise, within a Promise. Is there someway I can structure this better so that it is more straightforward? It seems like nesting them within each other is counter to Promise's intended chaining approach.

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 something until that Promise is plete. Something like this:

let promise = new Promise((resolve, reject) => {

  // Do some stuff

});

doSomethingUntilPromiseisDone(promise);

However, within my Promise, I need to execute another method that returns another Promise:

let promise = new Promise((resolve, reject) => {

  fetchValue(url)
    .then((value) => {

      // Do something here

    }).catch((err) => {
      console.error(err);
    });

});

doSomethingUntilPromiseisDone(promise);

But now, in the fetchValue method's then statement, I have another method I need to execute that, guess what, returns another Promise:

let promise = new Promise((resolve, reject) => {

  fetchValue(url)
    .then((value) => {

      saveToCache(value)
        .then((success) => {

          console.log('success!!');
          resolve('success');

        });

    }).catch((err) => {
      console.error(err);
    });

});

doSomethingUntilPromiseisDone(promise);

So in the end, I have a Promise, within a Promise, within a Promise. Is there someway I can structure this better so that it is more straightforward? It seems like nesting them within each other is counter to Promise's intended chaining approach.

• javascript
• node.js
• promise

Share Improve this question Follow edited Oct 25, 2016 at 15:01 Jake Wilson asked Oct 25, 2016 at 5:43 Jake WilsonJake Wilson 91.4k9797 gold badges260260 silver badges371371 bronze badges 6

• Looks like you are trying to code async calls backs with Promise -- why not just use callbacks instead? – Soren Commented Oct 25, 2016 at 5:45
• Where do you call resolve()? – guest271314 Commented Oct 25, 2016 at 5:51
• 1 Where is the got method? – thefourtheye Commented Oct 25, 2016 at 6:05
• Sorry the got was a typo. I was referring to the fetchValue method. – Jake Wilson Commented Oct 25, 2016 at 14:21
• @Soren Well, doSomethingUntilPromiseIsDone takes a Promise. fetchValue returns a Promise, and savetoCache returns a Promise. If Promises are what these methods are returning how would I use callbacks at all? – Jake Wilson Commented Oct 25, 2016 at 14:22

 |  Show 1 more ment

3 Answers 3

Sorted by: Reset to default 5

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))

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();

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);