How can I sequentially chain promises using bluebirdjs?

Question

Promise.all() doesn't guarantee that promises will be resolved in order. How can this be done?

Pinochle

Answer 1

Since you're using Bluebird JS, this can be actually done in a simple way.

In version 2.0, Bluebird introduced the Promise.each method that does this, for looping a then is simple enough, but since it is so common and got requested time after time eventually it was added as its own method.

function foo(item, ms){ // note bluebird has a delay method
    return Promise.delay(ms, item).then(console.log.bind(console))
}

var items = ['one', 'two', 'three'];

Promise.each(items, function(item, i){
    return foo(item, (items.length - i) * 1000)
});

Which produces the same result as the other answer, only with less lines of code and it also lets Bluebird perform optimizations on the iteration.

Quiescent

Answer 2

The thing that confused me most is that the async function being chained needs to return a function that returns a promise. Here's an example:

function setTimeoutPromise(ms) {
  return new Promise(function (resolve) {
    setTimeout(resolve, ms);
  });
}

function foo(item, ms) {
  return function() {
    return setTimeoutPromise(ms).then(function () {
      console.log(item);
    });
  };
}

var items = ['one', 'two', 'three'];

function bar() {
  var chain = Promise.resolve();
  items.forEach(function (el, i) {
    chain = chain.then(foo(el, (items.length - i)*1000));
  });
  return chain;
}

bar().then(function () {
  console.log('done');
});

Notice that foo returns a function that returns a promise. foo() does not return a promise directly.

See this Live Demo

Pinochle

Why are you returning a chain.then() when then() is called like that it effectively doesn't do anything but copy the promise. – Quiescent

Benjamin's example is definitely simpler. I developed mine as I was dealing with the lie promise lib (github.com/calvinmetcalf/lie) in pouchdb in which case there isn't support for the each() call. My question should really have referenced liejs and not bluebirdjs :). – Pinochle

@BenjaminGruenbaum how else would you chain these promises using my code? I'm still learning the nuances of promises, so I'd appreciate your input. – Pinochle

return chain instead of return chain.then() – Quiescent

Yup, missed that one :) – Pinochle

Also, for .. in loops do not guarantee iteration order, this can be result in an incorrect order in your case, you can use items.forEach or for(var i = 0;... instead – Quiescent