Quick promise example

promise-example.js

var Q = require("q");
var FS = require("fs");

function process_file(fn) {
  var deferred = Q.defer();
  
  FS.readFile(fn, "utf-8", function(error, content) {
    if (error) {
      deferred.reject(new Error(error));
    }
    
    // do processing to content
    
    deferred.resolve();
  });

  return deferred.promise;
}

Q.all([
  process_file(file_a),
  process_file(file_b),
  process_file(file_c)
])
  .then(function(results) {
    console.log("All calls completed without error");
  })
  .fail(function(error) {
    console.error(error);
  });