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?

Answer 1

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