httpBackend.js

var XHR = window.XMLHttpRequest || function() {
  try { return new ActiveXObject("Msxml2.XMLHTTP.6.0"); } catch (e1) {}
  try { return new ActiveXObject("Msxml2.XMLHTTP.3.0"); } catch (e2) {}
  try { return new ActiveXObject("Msxml2.XMLHTTP"); } catch (e3) {}
  throw new Error("This browser does not support XMLHttpRequest.");
};


/**
 * @ngdoc object
 * @name ng.$httpBackend
 * @requires $browser
 * @requires $window
 * @requires $document
 *
 * @description
 * HTTP backend used by the {@link ng.$http service} that delegates to
 * XMLHttpRequest object or JSONP and deals with browser incompatibilities.
 *
 * You should never need to use this service directly, instead use the higher-level abstractions:
 * {@link ng.$http $http} or {@link ngResource.$resource $resource}.
 *
 * During testing this implementation is swapped with {@link ngMock.$httpBackend mock
 * $httpBackend} which can be trained with responses.
 */
function $HttpBackendProvider() {
  this.$get = ['$browser', '$window', '$document', function($browser, $window, $document) {
    return createHttpBackend($browser, XHR, $browser.defer, $window.angular.callbacks,
        $document[0], $window.location.protocol.replace(':', ''));
  }];
}

function createHttpBackend($browser, XHR, $browserDefer, callbacks, rawDocument, locationProtocol) {
  // TODO(vojta): fix the signature
  return function(method, url, post, callback, headers, timeout, withCredentials, responseType, hooks) {
    var status;
    $browser.$$incOutstandingRequestCount();
    url = url || $browser.url();

    if (lowercase(method) == 'jsonp') {
      var callbackId = '_' + (callbacks.counter++).toString(36);
      callbacks[callbackId] = function(data) {
        callbacks[callbackId].data = data;
      };

      var jsonpDone = jsonpReq(url.replace('JSON_CALLBACK', 'angular.callbacks.' + callbackId),
          function() {
        if (callbacks[callbackId].data) {
          completeRequest(callback, 200, callbacks[callbackId].data);
        } else {
          completeRequest(callback, status || -2);
        }
        delete callbacks[callbackId];
      });
    } else {
      var xhr = new XHR();
      xhr.open(method, url, true);
      forEach(headers, function(value, key) {
        if (value) xhr.setRequestHeader(key, value);
      });

      // Add listeners for events such as progress
      hooks = hooks || {};
      var uploadHooks = hooks.upload || {};
      delete hooks.upload;
      forEach(Object.keys(hooks), function(event) {
        xhr.addEventListener(event, hooks[event], false);
      });
      forEach(Object.keys(uploadHooks), function(event) {
        xhr.upload.addEventListener(event, uploadHooks[event], false);
      });

      // In IE6 and 7, this might be called synchronously when xhr.send below is called and the
      // response is in the cache. the promise api will ensure that to the app code the api is
      // always async

      xhr.onreadystatechange = function() {
        if (xhr.readyState == 4) {
          var responseHeaders = xhr.getAllResponseHeaders();

          // TODO(vojta): remove once Firefox 21 gets released.
          // begin: workaround to overcome Firefox CORS http response headers bug
          // https://bugzilla.mozilla.org/show_bug.cgi?id=608735
          // Firefox already patched in nightly. Should land in Firefox 21.

          // CORS "simple response headers" http://www.w3.org/TR/cors/
          var value,
              simpleHeaders = ["Cache-Control", "Content-Language", "Content-Type",
                                  "Expires", "Last-Modified", "Pragma"];
          if (!responseHeaders) {
            responseHeaders = "";
            forEach(simpleHeaders, function (header) {
              var value = xhr.getResponseHeader(header);
              if (value) {
                  responseHeaders += header + ": " + value + "\n";
              }
            });
          }
          // end of the workaround.

          // responseText is the old-school way of retrieving response (supported by IE8 & 9)
          // response and responseType properties were introduced in XHR Level2 spec (supported by IE10)
          completeRequest(callback,
              status || xhr.status,
              (xhr.responseType ? xhr.response : xhr.responseText),
              responseHeaders);
        }
      };

      if (withCredentials) {
        xhr.withCredentials = true;
      }

      if (responseType) {
        xhr.responseType = responseType;
      }

      xhr.send(post || '');
    }

    if (timeout > 0) {
      var timeoutId = $browserDefer(timeoutRequest, timeout);
    } else if (timeout && timeout.then) {
      timeout.then(timeoutRequest);
    }


    function timeoutRequest() {
      status = -1;
      jsonpDone && jsonpDone();
      xhr && xhr.abort();
    }

    function completeRequest(callback, status, response, headersString) {
      // URL_MATCH is defined in src/service/location.js
      var protocol = (url.match(SERVER_MATCH) || ['', locationProtocol])[1];

      // cancel timeout and subsequent timeout promise resolution
      timeoutId && $browserDefer.cancel(timeoutId);
      jsonpDone = xhr = null;

      // fix status code for file protocol (it's always 0)
      status = (protocol == 'file') ? (response ? 200 : 404) : status;

      // normalize IE bug (http://bugs.jquery.com/ticket/1450)
      status = status == 1223 ? 204 : status;

      callback(status, response, headersString);
      $browser.$$completeOutstandingRequest(noop);
    }
  };

  function jsonpReq(url, done) {
    // we can't use jQuery/jqLite here because jQuery does crazy shit with script elements, e.g.:
    // - fetches local scripts via XHR and evals them
    // - adds and immediately removes script elements from the document
    var script = rawDocument.createElement('script'),
        doneWrapper = function() {
          rawDocument.body.removeChild(script);
          if (done) done();
        };

    script.type = 'text/javascript';
    script.src = url;

    if (msie) {
      script.onreadystatechange = function() {
        if (/loaded|complete/.test(script.readyState)) doneWrapper();
      };
    } else {
      script.onload = script.onerror = doneWrapper;
    }

    rawDocument.body.appendChild(script);
    return doneWrapper;
  }
}