when to use $q.defer() in Angularjs
<p>I am new to Angularjs and I m following this tutorial :<a href="http://mherman.org/blog/2015/07/02/handling-user-authentication-with-the-mean-stack/#.WE70iubhCM8" rel="nofollow noreferrer">http://mherman.org/blog/2015/07/02/handling-user-authentication-with-the-mean-stack/#.WE70iubhCM8</a> . However i dont understand when to use  $q.defer(). For example in the folllowing Angularjs code why use  $q.defer() :</p>

<pre><code>function login(username, password) {

      // create a new instance of deferred
      var deferred = $q.defer();

      // send a post request to the server
      $http.post('/user/login',
        {username: username, password: password})
        // handle success
        .success(function (data, status) {
          if(status === 200 &amp;&amp; data.status){
            user = true;
            deferred.resolve();
          } else {
            user = false;
            deferred.reject();
          }
        })
        // handle error
        .error(function (data) {
          user = false;
          deferred.reject();
        });
</code></pre>

<p>The server side code is :</p>

<pre><code>router.post('/login', function(req, res, next) {
  passport.authenticate('local', function(err, user, info) {
    if (err) {
      return next(err);
    }
    if (!user) {
      return res.status(401).json({
        err: info
      });
    }
    req.logIn(user, function(err) {
      if (err) {
        return res.status(500).json({
          err: 'Could not log in user'
        });
      }
      res.status(200).json({
        status: 'Login successful!'
      });
    });
  })(req, res, next);
});
</code></pre>

<p>And why not use it in the following Angularjs code :</p>

<pre><code> function getUserStatus() {
      return $http.get('/user/status')
      // handle success
      .success(function (data) {
        if(data.status){
          user = true;
        } else {
          user = false;
        }
      })
      // handle error
      .error(function (data) {
        user = false;
      });
    }
</code></pre>

<p>Server side code is :</p>

<pre><code>router.get('/status', function(req, res) {
  if (!req.isAuthenticated()) {
    return res.status(200).json({
      status: false
    });
  }
  res.status(200).json({
    status: true
  });
});
</code></pre>