Promise example with When.js (https://github.com/cujojs/when)

gistfile1.js

// It's pretty straightforward to wrap callback-based functions in ones that
// use and return promises.
function readOne (url) {
   var deferred = when.defer();
   httpRequest(url, function(response){
      deferred.resolve(response.length);
   });
   // This promise represents the end result of the operation, and will
   // be resolved when `httpRequest` calls `deferred.resolve` above.
   // Once resolved, anything waiting for its result can continue.
   return deferred.promise;
}

function sumAll (array) {
   var sum = array.reduce(function(m,n) { return m + n });
   // Do something with `sum`.
}

var url_array = ["http://scripting.com", "http://google.com", "http://twitter.com"];

// Call the request routine on each URL, collecting the promises into an
// `Array`. When all the promises are resolved, call `sumAll` with the results
// of each `readOne` call in an `Array`. (Note how the actual code is nearly
// the above comment, verbatim.)
var active_requests = url_array.map(readOne);
when.all(active_requests).then(sumAll);
// Code here will be called after the requests are initiated, but before
// `sumAll` is called.

// …or…

// Many promise libraries have utilities to consolidate those kinds of
// array operations.
when.map(url_array, readOne).then(sumAll);