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Promises in AngularJS

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Promises in AngularJS


Promises(約束) are objects which were proposed by ECMAScript 6. Their purpose is to run asynchronous operations. For example, if the part of a code needs a certain resource, it could load it in a promise, and resume the execution of this part of the code once the loading has completed. A promise can have three states:

  • :speech_balloon: Pending: The promise is still executing.
  • :o: Fulfilled: The promise has successfully completed.
  • :x: Rejected: The promise has completed with an error.

Implementation: The $q service

In AngularJS, promises are created using a service named $q. It is based on Q, an independant JavaScript library.


Let's say we have a resource which takes some time to load. Instead of waiting for the loading to finish, we're going to load it asynchronously and only display it when it is available.

Create the promise

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

    // Timeout to simulate the loading
    $timeout(function() {
        var resource = "Resource content";
        if(resource != null) {
            // Fulfilled: The promise has successfully completed.
        else {
            // Rejected: The promise has completed with errors.
            deferred.reject("An error occured while loading the resource.")
    }, 1000);

    return deferred.promise;

Use the promise

$scope.loadResource = function() {
    $scope.resource = "Loading..."
    var promise = createLoadPromise();
    promise.then(function(resource) {
        $scope.resource = resource;                   
    }, function(error) {
        $scope.resource = "";                   
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