Better User Experience with Promises in AngularJS

February 9, 2015

We live in the golden age of software development. Writing mobile and web applications is becoming easier thanks to amazing and brand new technologies and thanks to growing communities around them. Frameworks like Rails and AngularJS (just to name a few) provides us the power to build a working prototype in hours, if not minutes sometimes. But along with technologies, users needs grow too. I remember the days when I first started doing web development there was a simple rule: “If you make a user wait more than 30 seconds on a page, he/she will go away”.

30 seconds. I can’t believe it. Now we talk about milliseconds. If you can’t provide instant feedback to the user he/she will complain. Your clients will have unhappy customers, and you will have unhappy clients. Everyone will be unhappy!

Users will trigger lots of blocking operations on your application. These are operations that your application depends on and that take long time to complete. Like, for example, an image upload with thumbnails generation.

This is exactly what I’ll be talking about on this post.

My Experience

I had to implement, as part of a bigger application, a system that provided the user the ability to add custom images to a racing suit. The user could then manipulate those images (resize and rotate) and submit his/her design. Users could potentially load really big images and I couldn’t make them wait for the server to do the upload and the thumbnails generation.

Promises to the rescue

I solved the problem using Promises. This is not a post about promises but I promise I’ll go into details about them in a future post…


This is a simple list of the tasks to be implemented for the feature:

  1. Upload multiple images via drag & drop (one at a time)
  2. Manage uploaded images through a “layers” list (kinda like the one on Photoshop)
  3. Select an image
  4. Remove the selected image
  5. Resize/Rotate the images
  6. Save the changes to the canvas

Actually each of these task can be seen as an individual feature, because some of them are really complex (at least for me). For example, task n 1 involves all these subtasks:

  1. Upload multiple images via drag & drop (one at a time)
  2. Take a dropped image
  3. Encode the image in base64 so it can be used as soon as possible into the canvas (see point 3 above)
  4. Upload the image to the backend
  5. Add a new layer to layers list with a thumbnail of the image

For tasks 1.1 and 1.3 I’ve used a really cool library called angular-file-upload by Danial Farid that takes care of the drag & drop and the file upload.

After the user drops the file, the app uploads the image to the server which stores the image and generates a thumbnail. This thumbnail is later used in the app to show a preview on the layer.

The library exposes an upload method that returns a promise. So we can start the upload, save the promise in a variable and pass it around to the pieces of code that will need the thumbnail. The library exposes also an handy API you can use to append callbacks to the promises when, for example, it get resolved or rejected (respectively success and error).

While the server is doing its stuff, the application encodes the image in base64, creates the layer, adds the base64 encoded image to the canvas. This way we provide instant feedback to the user so he/she does not have to wait for the server to save the image and generate the thumbnail. As soon as the user drops the image, he/she can begin to manipulate the image and can see the new layer in the layers list.

Later on the server will send the thumbnail to the js app which will update the relative preview image in the layers list.

Here’s an approximation of the code, with all the irrelevant bits left out.

<div ng-controller="ImageCtrl">
  <div class="ddarea" ng-file-drop="onFileDropped($files)" ng-file-drag-over-class="dropping">
    <p>Drag & Drop your design image here</p>

  <div class="layers">
    <div class="layer" ng-class="{ active : }" ng-repeat="layer in layers">
      <div style="background-image: url({{layer.thumb}})"></div>
      <a ng-click="removeLayer(layer)">Remove</a>

  <div class="canvas">
.service('FileUploaderService', ['$upload', function($upload) {
  var _upload: function(file) {
    return $upload.upload({
      url: '/upload',
      method: 'POST',
      file: file

  return {
    upload: _upload
.controller('imageCtrl', [function() {
  $scope.layers = [];
  $scope.onFileDropped = function($files) {
    var file = $files[0];
    if (file !== undefined) {
      var uploadPromise = FileUploaderService.upload(file);
      uploadPromise.error(function(response) {
        $scope.notify("Upload error: " + response.message);

      var fileReader = new FileReader;

      fileReader.onload = function() {
        var layer = { 
          active: false,
          image = fileReader.result
        $scope.createNewLayer(layer, uploadPromise);
        $scope.addImageToCanvas(layer, uploadPromise);

  $scope.createNewLayer = function(layer, uploadPromise) {
      layer.thumb = response.thumb;
    uploadPromise.error(function(response) {
      $scope.layers.splice(_.indexOf($scope.layers, layer), 1);

  $scope.addImageToCanvas = function(layer) {
    // code to add the image to the canvas
    uploadPromise.error(function(response) {
      // Remove the layer from the canvas

The FileUploaderService is a wrapper around the $upload module provided by the angular-file-upload library. It exposes an upload method that returns the promise.

On line 20 of the image.js file we start the file upload and store the promise into the uploadPromise variable. We immediately append a function for when the promise is rejected (if there’s a problem with the file upload like network error, or validation errors) and we notify the user.

After encoding the image on line 27, we call the createNewLayer and the addImageToCanvas methods passing them the promise.

The createNewLayer method renders the new layer immediately – it simply pushes the new layer to the layers array – and uses the promise to take the thumbnail generated by the server. Once the promise is resolved, it adds the thumbnail to the layer object. Since the layers array is bounded to the view, AngularJS will automatically redraw the layer with the generated thumbnail.

The addImageToCanvas method uses and external library to generate an image object from the base64 encoded image and appends it to the canvas. The code is left out because it is not the focus of this post. For the sake of discussion the library I’ve used is FabricJS.

Both methods implement behaviour for when the promise is rejected. The createNewLayer method removes the layer from the layers lists, and the addImageToCanvas method removes the image from the canvas.

Advantages of using promises

The thing I like the most about promises is that you can instantiate one, pass it around, and append it as many then as you like. Your promise will only get resolved (or rejected) once and all your thens will get called. Also, you can append a then even if the promise has already been resolved. Your then will be called.

There are other cool stuff promises provides. Like the ability to wait for more than one promise to resolve, or the ability to chain promises, but these will be topics for future posts.

Until then, happy coding!