async and Q promises in nodejs

Question

I'm using the Q library and async library in nodejs.

Here's an example of my code:

async.each(items, cb, function(item) {

 saveItem.then(function(doc) {
    cb();
 });

}, function() {

});

saveItem is a promise. When I run this, I always get cb is undefined, I guess then() doesn't have access. Any ideas how to work around this?

Autolysin

Answer 1

Your issue doesn't lie with promises, it lies with your usage of async.

async.each(items, handler, finalCallback) applies handler to every item of the items array. The handler function is asynchronous, i.e. it is handed a callback, that it must call when it has finished its work. When all handlers are done, the final callback is called.

Here's how you'd fix your current issue:

var handler = function (item, cb) {
  saveItem(item)
  .then(
    function () { // all is well!
        cb();
    },
    function (err) { // something bad happened!
        cb(err);
    }
  );
}

var finalCallback = function (err, results) {
  // ...
}

async.each(items, handler, finalCallback);

---

However, you don't need to use async for this particular piece of code: promises alone fill this job quite nicely, especially with Q.all():

// Create an array of promises
var promises = items.map(saveItem);

// Wait for all promises to be resolved
Q.all(promises)
.then(
    function () { // all is well!
        cb();
    },
    function (err) { // something bad happened!
        cb(err);
    }
)

Palpate

Great you mentioned Q.all solution – Eames

Hot code. Will definitely be using this for fun and profit. :) – Dunite