Mocking API calls in Aurelia

Aurelia is a modern and slick single-page application framework. “Single-page application” aspect means that it’s loaded into the browser once, and then the navigation happens on the client side and all the data are loaded from a REST API endpoint.

Let’s say that our front-end Aurelia app is hosted at while the REST API is hosted at The REST API is a server-side application, which can be implemented in .NET, Java, Node.JS etc., and it talks to a database of some kinds.

For the front-end development purpose, it’s usually useful to be able to mock the connection to API with some static manually generated data. This cuts the hard dependency between the client code, the backend code and database. It’s much easier to mock the exact data set which is needed for the current development task.

Fortunately, it can be easily done, and here is how.

Identify your requests

Create a list of the requests that you need to mock. For our example let’s say you do the following requests from the application:

GET /api/products
GET /api/products/{id}
POST /api/products

Put your mock data into files

Go to the root folder of your Aurelia app and create an /api folder.

Create a /api/products subfolder and put a new file called GET.json. This file should contain the JSON of the product list, e.g.

[ { "id": 1, "name": "Keyboard", "price": "60$" },
  { "id": 2, "name": "Mouse", "price": "20$" },
  { "id": 3, "name": "Headphones", "price": "80$" }

Create a new file called POST.json in the same folder. POST response won’t return any data, so the file can be as simple as


Create subfolders 1, 2 and 3 under products and create a GET.json file in each of them. Every file contains the data for a specific product, e.g.

{ "id": 1, 
  "name": "Keyboard", 
  "price": "60$",
  "category": "Computer Accessories",
  "brand": "Mousytech"

Configure BrowserSync to mock your API calls

For the purpose of this post, I assume you are using Aurelia Skeleton Navigation starter kit, specifically the version with Gulp-based tasks and BrowserSync. If so, you should be familiar with gulp serve command, which serves your application at http://localhost:9000. We will extend this command to host your API mock too.

Navigate to /build/tasks folder and edit the serve.js file. Change the definition of serve task to the following code:

gulp.task('serve', ['build'], function(done) {
    online: false,
    open: false,
    port: 9000,
    server: {
      baseDir: ['.'],
      middleware: function(req, res, next) {
        res.setHeader('Access-Control-Allow-Origin', '*');

        // Mock API calls
        if (req.url.indexOf('/api/') > -1) {
          console.log('[serve] responding ' + req.method + ' ' + req.originalUrl);
          var jsonResponseUri = req._parsedUrl.pathname + '/' + req.method + '.json';
          // Require file for logging purpose, if not found require will 
          // throw an exception and middleware will cancel the retrieve action
          var jsonResponse = require('../..' + jsonResponseUri);
          // Replace the original call with retrieving json file as reply
          req.url = jsonResponseUri;
          req.method = 'GET';

  }, done);

Run it

Now just run gulp serve (or gulp watch, which does serve and then watches files for changes). Every time your app makes an API call, you will see a line in the gulp console:

[serve] responding GET /api/products

If you happen to make an invalid request with no mock defined, you will get an error:

[serve] responding GET /api/notproducts
Error: Cannot find module '../../api/notproducts/GET.json'

A complete example can be found in my github repository.

Cloud developer and researcher.
Software engineer at Pulumi. Microsoft Azure MVP.

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