Writing Integration Tests for WebSocket Servers Using Jest and WS

Isaiah Thomason
11 min readOct 27, 2020

WebSockets are very useful for ongoing communication between a client and a server. They’re simple to use in nature, but they’re not so simple when it comes to writing tests. This is because WebSockets are event-driven and have no promise-based API. For instance, maybe you want to test that your WebSocket server returns the correct message to a client with Jest. How will you wait for a connection before having your test client send a message? How will you get a hold of the message your client received and perform assertions? How will Jest know when a given test is finished? These are the kinds of questions I hope to address in this post on writing integration tests for WebSocket servers.

Here’s our outline:

Note that Covering More Test Cases is by far the longest section here and is completely optional. It’s only necessary if you want more complex examples.

Everything here can also be found on github.


Before we get started, we’ll need to install the necessary packages. We’ll be using jest for our tests and ws for our web socket server. You’re free to use different tools, but you’ll have to adjust your syntax accordingly as you go through the examples.

npm install ws
npm install -D jest

Project Setup

Before we can do anything, we need an actual WebSocket server to test. Let’s create a function that makes one.

This function creates a WebSocket server from the server you pass to it. It’s particularly useful because it allows you to use your real server when running your application and a simple one when running tests. To keep things simple, we’re only echoing back whatever the client sends. We’ll update this later!

Next, let’s set up our test file. We’ll just start with a skeleton. We know we’ll need to start the server before all our tests, we know we’ll need to close the server after all our tests, and we know we’ll need a physical test for our WebSocket server. Let’s start with that.

Now that we have a roadmap of what we need, let’s start filling in the blanks!

Creating Utility Functions

It might seem weird to have this as its own section, but honestly, this is arguably the hardest part of writing integration tests for WebSocket servers. As I mentioned, we don’t have any out-of-the-box promise-based APIs for WebSockets. This means it’s critical to setup good utility functions to ensure that everything clearly happens in the right order. Otherwise, we’ll be tortured by tons of callbacks.

Here’s what we need:

  1. An awaitable function that starts the server and returns it
    * This is necessary for the beforeAll and afterAll portions of our test file.
  2. A function that can wait for a client to open or close a connection
    * Reliably sending test messages, performing assertions on the responses, and telling Jest when the test is done requires us to have this control.

We’ll walk through each one of these functions and apply them to our test. Let’s put these utility functions in a separate file called webSocketTestUtils.js.

First Utility: Start Server Function

This one should be pretty straightforward. Here’s the code that we’ll use:

Here, we’re merely creating (and starting) a basic server that we can use to test all of our WebSocket functionality. For modularity, the port number to listen on is passed in. This function will be easy to use in our test file.

Second Utility: Function to Wait for Socket State

This function is another easy one, but it requires a good grasp on how promises work. I’ll “comment out” some of the code we’re not focused on for brevity.

In short, this function takes a client WebSocket and forces the process to wait until the client socket’s state becomes the desired value. It does this by recursively calling itself until the client socket’s state is correct.

The setTimeout function acts as a light buffer to avoid performing too many function calls. You can make the delay whatever you want, but I recommend using a small number to verify that the function behaves properly. (If the function behaves incorrectly and the delay is small, Jest will log warnings/errors due to async-related problems.)

Writing the Integration Test

With the basic utility functions done, we can finally start writing our first integration test! We’ll follow the process that we put in the comments earlier: 1) Create the test client, 2) Send the client message, 3) Close the client after it receives the response, and 4) Perform assertions on the response.

Let’s walk through this. We start off by creating a test client and waiting for its connection to open. We then setup some variables that we’ll be using for our test.

Next, we setup the client to handle messages. The response that the client receives is saved so that we can perform assertions on it later. Once the client receives the response, we close it since we no longer need to have it open. In Jest, you should always close clients when you’re finished with them to avoid errors.

After that, we have our client send the test message to our WebSocket server. It’s safe to do this since we waited for an open connection.

Finally, we wait for the client socket to close before performing our assertions. Note that waiting for the socket to close is critical, as we can’t be guaranteed that we have all the messages we need until we know the client is finished.

And that’s it! You can verify that the test succeeds by running npx jest. Alternatively, you can make an npm script that runs jest for you.

Adding One More Utility for the Client

If you didn’t notice from the earlier example, setting up a client without a utility function can easily get verbose and redundant as you add more tests of complex variety. There are some additional test cases worth considering, but before we dive into those, we should simplify the process of setting up a test client. We’ll add these changes to webSocketTestUtils.js.

This function creates a new client socket that connects to the specified port. It then waits for the socket connection to open before setting up an event handler.

The event handler adds any messages it receives to an array, and it closes the client when the expected number of messages — denoted by closeAfter -- is received. Writing the handler this way gives us flexibility: If we're only expecting N messages and we want the client to close when it's finished, then we can call createSocketClient(port, N). Alternatively, if we're not waiting for an explicit number of messages, we can omit closeAfter and supply the closing logic in our test. We'll see the benefit of this later.

The function returns the client along with any messages the client receives. This enables the test using this function to control the client’s behavior and perform assertions on all of the responses.

Let’s apply our new changes to the test we wrote earlier:

This looks so much cleaner! If you add a new developer to your team and they see this code for the first time, it will read much more like plain English since the complications of callbacks and event handlers are abstracted away. Adding documentation to the testing utilities would further improve the developer experience, but I’ll leave that as “extra credit”.

Now about those additional test cases I mentioned…

Covering More Test Cases (Optional)

It’s great that we can test messages echoed from the WebSocket server, but that doesn’t give us much to work with. What if the server is supposed to respond to 1 client message with multiple messages? What if the server is supposed to send a message to multiple clients? We need ways to test these conditions. And that’s what we’ll look at next.

Note: If you feel you’ve read all you need to get started, you can skip this entire section! It’s the largest one here. Otherwise, we’ll be consider 3 more test cases before wrapping up:

  1. The server sending multiple messages back to the client.
  2. The server sending a message to multiple clients.
  3. The server sending a message to multiple specific clients.

Writing a test case for our WebSocket server requires our server to have code that handles said case to begin with. So as we go through each test case, we’ll first update createWebSocketServer.js and then update createWebSocketServer.test.js.

First New Test: Having the Server Send Multiple Messages

I want to refactor our createWebSocketServer function a little bit. Since we'll be handling multiple scenarios, we need the WebSocket server to know how to handle different kinds of messages. There are multiple ways to approach this problem. One way is to tell the server to expect an object with a type property and a value property. The type property will drive the behavior of the server, and the value property will specify the intended content of the message. For our small example, this is fine. We'll update our original code first.

Notice that we’re expecting the object to come in as a JSON string that we can parse. Again, this is just one of many approaches.

Next, let’s add something that causes the server to send multiple responses to the same client.

In the real world, you’d probably be sending different kinds of messages instead of repeating the same one. But for the sake of our example, this will suffice. Let’s update our test file next. Note that we’ll have to make changes to the first test and then add our second test.

You can see that our new test is not too different from our first one. This is thanks to the utility functions we created earlier!

Second New Test: Having the Server Send a Message to Multiple Clients

For this section, we’ll have the server echo the client’s message to everyone who’s connected. This code change is straightforward. I’ll focus only on the switch/case statement here for brevity.

And now we add our test. Again, I’ll focus solely on the new code for brevity.

Yet again, our new test is fairly similar to our old ones. We just had to add more clients. Notice that due to the nature of the test, only 1 client needed to send a message.

The value of our utility functions can’t be missed here: Without createSocketClient, we'd be repeating a lot of the same code several times. And if we weren't using a promise-based utility like waitForSocketState, we'd have to use callbacks in every place where we wait for a given socket state. This includes all calls that appear in createSocketClient, which means that trying to reconcile starting and closing all the clients at the proper time would become a nightmare; we'd end up with lots of confusing callback nesting. I meant it when I said the utility functions were the most significant part here.

Third New Test: Having the Server Send a Message to Multiple Specific Clients

This example is a little more ambitious. We’ll be creating a fake group chat. Clients who connect to the WebSocket server will be able to create a group, join a group, and send a message to their group. When a message is sent to a group, only the clients in that group (including the sender) will receive the message. This will require adding 3 additional cases to our switch/case statement from earlier.

Here, we’ve added a groupNames array to keep track of all the groups that currently exist. When a person creates a new group, it's added to groupNames. If the group's name is taken, the client gets an error message. Clients seeking to join a group must use a name that is already in groupNames. Invalid names will result in an error message from the server. Successfully joining/creating a group will return the requested group name as confirmation of success.

Finally, whenever a group message is sent, the message is returned to all clients associated with that group name. The message will only go through if it was sent by a client who was already in the group.

Remember that this is an example for the sake of showing potential ways to write tests. A more realistic WebSocket server would be more complex. For instance, it would provide a unique identifier for each group, delete a group when no more clients are associated with it, and more.

We have enough to get us going, so we can finally write our last test now. The trick here is keeping track of the order of events. A client can’t join a group that doesn’t exist, so we need to make sure that anyone trying to join a group does so after it is created. Let’s see what a potential test could look like:

This test is still fairly similar to our previous ones. The only difference here is the addition of event handlers to control the order of events. There are a few things worth calling attention to.

First, as I mentioned earlier, it’s impossible to join a group that doesn’t exist. client1's event handler allows us to join a group and send messages to it after we know for sure that a group has already been created.

Second, we’ve chosen client2 as the "key pillar" of the test. It's responsible for joining a group and sending a message to everyone in the group. For us to be sure that the group message was only received by the correct clients, we should only close client1 and client3 after client2 is finished. Thus, instead of providing the closeAfter argument for client1 and client3, we let client2 be responsible for closing the other clients. This is the benefit of keeping our createSocketClient utility flexible.

Finally, client1 puts everything in motion by creating a group. After this, we do what we've always done: We wait for all the sockets to close and perform assertions on the responses we received.

You’ll notice that although we added a couple new features to our WebSocket server, we’ve only created a test for one scenario. I’ll leave testing the other scenarios as an exercise if you’re up for it. Most of the other ones are of similar or easier difficulty.

Brief Comments

I’ll quickly comment on 2 things before wrapping up.

1) Managing the Order of Your WebSocket Tests

If you read the optional portion of this article, you saw that there may be situations where you have to control the order in which your clients send messages. This is always doable if you set up event handlers, but try to do so as cleanly as possible.

2) Make Each of Your Tests Specific to the Message Type You’re Focusing on

Whenever you create an application involving some kind of group or lobby, your tests become heavily event dependent. For instance, before you can test sending a message to a group, it is first necessary for one client to create a group and for another client to join the group.

I strongly recommend against meshing test assertions in this situation. For instance, a test about sending a message to the group should not perform assertions on whether the group was correctly created or joined. Instead, create separate test cases for verifying that creating or joining a group works properly. Then, in your test about sending a message, you can focus solely on how the group message was handled.

Alternatively, you can create one large test that “goes through the entire flow” of creating, joining, and messaging a group. You get less code duplication, but you also get a larger test; so consider the trade-offs. WebSockets are still a beast since they are event driven, so every act of organization helps.


And now we’re finally done! In this article, we learned how to create integration tests for WebSocket servers, and we created a few utility functions to greatly simplify this process. Let me know what you thought! I want to make sure this is a sufficient example on writing integration tests for WebSocket servers, so all questions and critiques (and appreciations 😅) are welcome!

As I said at the beginning, this article and the code for its examples can be found on github. There, I have a JavaScript version and a TypeScript version of the codebase. Both use JSDocs to make life a little easier if you decide to play around.

I want to give a special thanks to stackoverflow users user3215378 and Dmitry Taipov for the inspiration I got for waitForSocketState, and to stackoverflow users FleMo and Timo Tijhof for the inspiration I got for createSocketClient. I also want to thank Kent C. Dodds for his insights into writing integration tests. If you're interested, you can find his courses on testing JavaScript here. He didn't touch on testing WebSocket servers, but he did provide the inspiration for me to try this after several headaches.