Building and testing an API with Express, Mongo, and Chai

Recently, I’ve been going through the Express, Mongoose, and Chai docs in order to help build out and test an API that’s going to be used for ACM Hack, a committee of UCLA’s CS club that focuses on teaching students new technologies and frameworks, as well as fostering/building an environment of hackers and makers at UCLA. We’re completely revamping Hack for the next quarter with regular events, projects, and additional content in terms of blog posts and tutorials for our users. To do this, we needed to revamp the Hack website.

Specifically, a few backend tasks were required, in the form of creating a functional API to support the needs of our front-end developers and users:

  • Create, update, get, and delete Events (an Event, for example, could be an “Android Workshop Session”)
  • Create, update, get, and delete Showcase Projects (these our projects that our hack members submit to us, and we showcase the coolest/most innovative projects)
  • Securing this API through the use of tokens, to make sure that requests cannot be spammed.
  • Create an email list API endpoint, that allows users to subscribe to our mailing list that notifies them about new events or important updates.
  • Create Mongoose schemas for all of the above data types.

Tools Used

On the backend, we decided to use MongoDB for our database, Express.js for our web framework, and Mocha/Chai for unit tests. The first order of business was to create database schemas for all of the above data types. We used mongoose to interact with our MongoDB database. Mongoose allows us to define object models that we can save and retrieve from our database. From the MongooseJS docs, models are compiled from their schema definitions and represent specific documents in our database. The models also handle document creation and retrieval.

To take the example of creating our mailing list API endpoint, it would be useful to have an email schema that contains both the user’s email address as well as the user’s name. Moreover, we’d like to be able to retrieve all emails in a single request. Here’s the schema that we defined for emails:

We defined a getAll function in our schema to support querying for the entire mailing list. From the MongooseJS docs, each model has find, findById, findOne and a few other useful functions that we can use to retrieve particular documents. We primarily used the find function, that has a few interesting use cases:

We used the latter to return all email documents, thus providing us with our mailing list.

Next, we created a mongoose instance and connected it to MongoDB. There are several ways to create your own MongoDB instance, a popular choice being MongoLab. We also exported our schemas so that they can be instantiated in other areas of our application, namely, in our API where these models will be created and accessed. The following code connects the mongoose instance and exports the schemas:

Defining Our API Endpoint with Express

The next step was to set up the Express framework and begin to define routes and endpoints for our application. Express is a minimal web framework that is essentially composed of two things: routing and middleware functions. At a high level, routing defines endpoints for your application that can be accessed to perform certain actions (ie, GET or POST certain data). In other words, it defines the structure that is used for interaction with the backend of your web app. An Express route essentially maps a URL to a specific set of functions, called middleware functions. Middleware functions are quite powerful, and are capable of the following actions:

  • Execute any code on the server
  • Modify the request (req) and response (res) object
  • Access the next middleware function on the stack, denoted by next()
  • End the API call.

For example, we can create a route for obtaining and sending data to our mailing list. To do this, we will create a router that maps the URL /api/v1/email/:email? to a set of functions. The last part of the URL, :email? is an an optional URL parameter. First, we can define middleware functions for this URL, which will also take care of the behavior of the endpoint without the optional argument:

In other files in our api directory of our application, we can tell Express to use certain routers for specific API endpoints. This way, routers can be composed: the /api endpoint can have routes for each API version, and each API version can have routes for its several endpoints that access data such as the mailing list or upcoming events:

//require the routers implemented for each data type
router.use(/event, require(./event).router);
router.use(`/email`, require(`./email).router);
router.use(`/showcase`, require(`./showcase).router);
module.exports = {router};
//require routers for each version of the API implemented
router.use(/v1, require(./v1).router); 
module.exports = {router};

With this setup, access to our application’s data was organized into several different API endpoints. Next, we had to actually implement each middleware function for each of our API endpoints. To do this, we had to think about our API’s design at a granular level: what fields will we require for particular requests? Which requests will need token authentication? What will the response body look like in the case of success and in the case of failure?

We decided that our response objects will have two high level fields: success, a boolean value that indicates the status of the request, and errors, a string that indicates the errors (if any) that were encountered during the request (such as an invalid ID or unauthorized token). Here’s an example implementation of a get request:

As indicated above, we can have certain requests require a valid token for the request to return successfully. Also, we pass in an anonymous function that takes in two parameters to the getAll function defined in our Email model. From the implementation of getAll in the email schema discussed previously, the function retrieves all emails and then calls a provided callback function. In this case, the function returns a response object back to the user.

Testing the API using Mocha and Chai

Next, we moved on to testing our API endpoints to make sure they work well, especially in edge cases such as malformed or unauthorized requests. At first, we manually tested our API using Postman, which is a useful tool for quickly querying your endpoint to make sure it works correctly. However, as our API and overall application began to change rapidly and increase in size, we decided to use unit testing in order to make sure that our core functionality doesn’t break as a result of an erroneous commit.

Unit tests allowed us to automatically detect problems in our codebase when they happen, and we can make sure we don’t push a broken build by making sure all of our tests pass during the build step. We used two JavaScript unit testing libraries: Mocha.js, which allows us to actually run unit tests, and Chai.js which contains several useful helper functions to write our testing code. Using a few more add-ons such as chai-Http (to create and send HTTP requests) and chai-should (to write clean assert statements), we can efficiently create a testing schematic for our API.

First, we describe a test and what it should do, and have an anonymous function running the actual test. The test for an API makes a request to that endpoint with some data, and then we verify that the response object looks like it should. As an example, to test our email API endpoint, we did the following:

  • Create a valid GET request with a valid token in the body. Verify that the response object contains the relevant status fields and returns mailing list.
  • Create an invalid GET request that is missing a valid token. Verify that the response object indicates failure and provides no emails.
  • Create a valid POST request that has a body indicating the user’s name and email address. Verify that the response object indicates that the request executed successfully.
  • Create a valid POST request that has a body that is missing optional fields. Ensure that missing these optional fields doesn’t cause the request to fail.

Here’s an example of a single test case:

To easily run our tests, we just need to add the line "test": "mocha" to our package.json file. Then, the unit tests can be run with a single command line argument: npm test. Chai and Mocha allow the developer to create and define tests so that the end result of running the tests is descriptive of what tests were run, and how they should behave:


And that’s it! We now have a well-organized, reliable, and reusable set up for creating and testing a robust API. In the coming months, we hope to expand on this and push out even more interesting features for UCLA’s CS community.

Project/Code Contributors:

Written on January 3, 2017