WebSocket Client with JavaScript

In the previous article of this series, "WebSocket with JavaScript and Bun", we explored how to initialize a server capable of handling both HTTP requests and WebSocket connections.

We defined a rule for HTTP requests to serve the index.html file when a request is made to /. The index.html file contains the client-side logic for establishing a connection with the WebSocket server and sending messages as a client.

The client code

In the fetch method of the server explained in "WebSocket with JavaScript and Bun" is implemented this code:

  if (url.pathname === "/") 
    return new Response(Bun.file("./index.html"));

This means that when a browser request is made to http://localhost:8080/, the content of the index.html file is sent to the browser.
The HTML will render a simple form with input text and a button and ship the logic for connecting to the WebSocket server as a client.

    
        <title>WebSocket with Bun and JavaScript</title>
        <script>
            let echo_service;
            append = function (text) {
                document
                    .getElementById("websocket_events")
                    .insertAdjacentHTML("beforeend", "<li>" + text + ";");
            };
            window.onload = function () {
                echo_service = new WebSocket("ws://127.0.0.1:8080/chat");
                echo_service.onmessage = function (event) {
                    append(event.data);
                };
                echo_service.onopen = function () {
                    append("? Connected to WebSocket!");
                };
                echo_service.onclose = function () {
                    append("Connection closed");
                };
                echo_service.onerror = function () {
                    append("Error happens");
                };
            };

            function sendMessage(event) {
                console.log(event);
                let message = document.getElementById("message").value;
                echo_service.send(message);
            }
        </script>
        <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/@picocss/pico@2/css/pico.min.css">
    

    
        <main>



<h2>
  
  
  Explaining the client code
</h2>

<p>This code creates a simple <strong>WebSocket client</strong> in a browser to interact with a WebSocket server. Here's a detailed explanation of its components:</p>


<hr>

<h3>
  
  
  The HTML structure
</h3>



<pre class="brush:php;toolbar:false">

    
        <title>WebSocket with Bun and JavaScript</title>
    
    
        <main>



<ul>
<li>The input field (<input>
</li>
<li>The submit button (<input type="button">): when clicked, it triggers the sendMessage(event) function to send the typed message to the server.</li>
<li>The messages/events log (<ul>
</ul>


<hr>

<h3>
  
  
  The JavaScript logic
</h3>

<h4>
  
  
  Initializing the WebSocket connection
</h4>



<pre class="brush:php;toolbar:false">window.onload = function () {
    echo_service = new WebSocket("ws://127.0.0.1:8080/chat");
    ...
};

• WebSocket("ws://127.0.0.1:8080/chat"): creates a new WebSocket connection to the server at 127.0.0.1 on port 8080, specifically the /chat endpoint.
• The variable echo_service holds the WebSocket instance, which facilitates communication with the server.

---

Handling WebSocket events

The WebSocket client has four main event handlers:

1. onopen (the connection is established)

  if (url.pathname === "/") 
    return new Response(Bun.file("./index.html"));

• The onopen function is triggered when the connection to the server is successfully established.
• It appends a message to the log saying, "? Connected to WebSocket!".

1. onmessage (a message is received)

    
        <title>WebSocket with Bun and JavaScript</title>
        <script>
            let echo_service;
            append = function (text) {
                document
                    .getElementById("websocket_events")
                    .insertAdjacentHTML("beforeend", "<li>" + text + ";</script>

"); }; window.onload = function () { echo_service = new WebSocket("ws://127.0.0.1:8080/chat"); echo_service.onmessage = function (event) { append(event.data); }; echo_service.onopen = function () { append("? Connected to WebSocket!"); }; echo_service.onclose = function () { append("Connection closed"); }; echo_service.onerror = function () { append("Error happens"); }; }; function sendMessage(event) { console.log(event); let message = document.getElementById("message").value; echo_service.send(message); }

Explaining the client code

This code creates a simple WebSocket client in a browser to interact with a WebSocket server. Here's a detailed explanation of its components:

---

The HTML structure

    
        <title>WebSocket with Bun and JavaScript</title>
    
    
        <main>



<ul>
<li>The input field (<input>
</li>
<li>The submit button (<input type="button">): when clicked, it triggers the sendMessage(event) function to send the typed message to the server.</li>
<li>The messages/events log (<ul>
</ul>


<hr>

<h3>
  
  
  The JavaScript logic
</h3>

<h4>
  
  
  Initializing the WebSocket connection
</h4>



<pre class="brush:php;toolbar:false">window.onload = function () {
    echo_service = new WebSocket("ws://127.0.0.1:8080/chat");
    ...
};

• The onmessage function is triggered whenever a message is received from the server.
• The server’s message (event.data) is appended to the event log using the append function.

1. onclose (the connection is closed)

   echo_service.onopen = function () {
       append("? Connected to WebSocket!");
   };

• The onclose function is triggered when the connection to the server is closed (e.g., the server disconnects).
• The function appends "Connection closed" to the event log.

1. onerror (an error is occurred)

   echo_service.onmessage = function (event) {
       append(event.data);
   };

• The onerror function is triggered when an error occurs during communication.
• The function logs "Error happens" to indicate the issue.

---

Sending messages to the server

   echo_service.onclose = function () {
       append("Connection closed");
   };

• The sendMessage function is called when the "Submit" button is clicked.
• document.getElementById("message").value: it retrieves the text entered by the user in the input box.
• echo_service.send(message): it sends the user’s message to the WebSocket server.

---

Logging events

   echo_service.onerror = function () {
       append("Error happens");
   };

• This utility function adds WebSocket events and messages to the

  list (id="websocket_events").

• insertAdjacentHTML("beforeend", "

• " text ";
"): inserts the given text as a new list item (
• ) at the end of the list.

---

Styling with PicoCSS

function sendMessage(event) {
    let message = document.getElementById("message").value;
    echo_service.send(message);
}

PicoCSS provides a lightweight and elegant styling for the page, ensuring the form and event log look polished without additional custom CSS.

---

The recap, how it works

1. When the page loads, the browser establishes a WebSocket connection with the server.
2. Upon successful connection, a message is logged saying, "? Connected to WebSocket!".
3. Users can type a message in the input box and click the "Submit" button. The message is sent to the WebSocket server.

---

Next Steps

This article explored how to implement a WebSocket client to communicate with a WebSocket server. In the previous article of this series, we focused on structuring a basic WebSocket server.

In the next article, we will explore WebSocket functionality further by implementing broadcasting logic. This feature allows messages from one client to be forwarded to all connected clients, making it essential for building real-time applications like chat systems, collaborative tools, or live notifications.

Stay tuned!