gistfile1.js

function foo()
{
  var promise = new Promise();
  my_async(args, function callback()
  {
    //do something
    ...
    if(error_happens)
    {
      promise.reject(error_value); //calls the 'error' function in promise.then()
    }
    else
    {
      promise.resolve(success_value); //calls the 'success' function on promise.then()
    }
  });
  return promise;
}

foo().then(function success(success_val)
{
  ...
}, 
function failure(error_val)
{
  ...
});

//this part of the code will likely be reached before functions inside 'then' are executed