Node.js: How do you handle callbacks in a loop?

Question

I'm using Node.js and Box SDK. My (failing!) code looks like this:

var connection = box.getConnection(req.user.login);
connection.ready(function () {
  connection.getFolderItems(0, null, function (err, result) {
    if (err) {
      opts.body = err;
    } else {
      opts.body = result;
      var a = [];
      for (var i=0; i < result.entries.length; i++) {
        connection.getFileInfo(result.entries[i].id, function (err, fileInfo) {
        if (err) {
          opts.body = err;
        } else {
          a.push(fileInfo);
        }
      });}
    }

In "procedural" terms, this is which I'm trying to do:

var connection= box.getConnection()
var items = connection.getFolderItems()
var itemList = new List
foreach (item in items) {
  connection.getFileInfo(item.id)
  itemList.add(item)
}
display(itemList)

My problem is that connection.getFolderItems() and connection.getFileInfo() are asynchronous - the "for" loop exits before all the results are returned.

Q: What is the best way in Node.js to 1) get the result of the first async call, 2) iterate through the list, making more async calls, and 3) process the results when everything is "done".

Q: Are promises a good choice here?

Q: Is done()/next() an option?

Q: Is there any "standard idiom" in Node.js for this kind of scenario?

Answer 1

Promises are a great idea, but you may want to take a look at the async module, specifically the collection handlers. It allows you to run async calls against a list of 'things' and gives you a place to run a method when all of the async calls are done. Don't know if this is better than promises, but options are always nice.

// Should give you a rough idea
async.each(items, function (item, callback) {
  connection.getFileInfo(result, callback);
}, function (err) {
  console.log('All done');
});

https://github.com/caolan/async#each