The Deferred API

A new instance of deferred is constructed by calling $q.defer().

The purpose of the deferred object is to expose the associated Promise instance as well as APIs that can be used for signaling the successful or unsuccessful completion, as well as the status of the task.

Methods

resolve(value) – resolves the derived promise with the value. If the value is a rejection constructed via $q.reject, the promise will be rejected instead.

reject(reason) – rejects the derived promise with the reason. This is equivalent to resolving it with a rejection constructed via $q.reject.

notify(value) – provides updates on the status of the promise’s execution. This may be called multiple times before the promise is either resolved or rejected.
Properties

promise – {Promise} – promise object associated with this deferred.

// for the purpose of this example let's assume that variables `$q` and `okToGreet`
// are available in the current lexical scope (they could have been injected or passed in).

function asyncGreet(name) {
  var deferred = $q.defer();

  setTimeout(function() {
    deferred.notify('About to greet ' + name + '.');

    if (okToGreet(name)) {
      deferred.resolve('Hello, ' + name + '!');
    } else {
      deferred.reject('Greeting ' + name + ' is not allowed.');
    }
  }, 1000);

  return deferred.promise;
}

var promise = asyncGreet('Robin Hood');
promise.then(function(greeting) {
  alert('Success: ' + greeting);
}, function(reason) {
  alert('Failed: ' + reason);
}, function(update) {
  alert('Got notification: ' + update);
});

This code example was taken from Angular Documentation page: https://docs.angularjs.org/api/ng/service/$q