A wrapper that applies a transform to jQuery.Deferred objects.

example.js

// Low level helper function for calling some API.
function callAPI(path, params) {
  return $.get("/api/url" + path, params);
}

// High-level semantic function for retrieving widgets. Leverages the underlying
// API function but itself returns a promise.
function getAllWidgets(filter) {
  var apiCall = callAPI("/widgets", $.extend(filter, { foo: "bar" }));

  return redefer(apiCall, function(response) {
    return response["outer"]["inner"]["widgets"];
  });
}

redefer.js

// Accepts a promise and a transformation function and returns a new
// Deferred that resolves with the result of the transformation applied.
function redefer(promise, fn) {
  var def = $.Deferred();
  
  promise.then(function() {
    def.resolve(fn(arguments));
  });

  return def;
}