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.

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

    <body>
        <main class="container">
            Message: <input value="Hello!" type="text" id="message" />
            <input
                type="button"
                value="Submit"
                onclick="sendMessage(event)"
            /><br />
            <ul id="websocket_events"></ul>
        </main>
    </body>
</html>

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

<!doctype html>
<html>
    <head>
        <title>WebSocket with Bun and JavaScript</title>
    </head>
    <body>
        <main class="container">
            Message: <input value="Hello!" type="text" id="message" />
            <input
                type="button"
                value="Submit"
                onclick="sendMessage(event)"
            /><br />
            <ul id="websocket_events"></ul>
        </main>
    </body>
</html>

The input field (<input id="message">): a text box where users can type a message to send via the WebSocket.
The submit button (<input type="button">): when clicked, it triggers the sendMessage(event) function to send the typed message to the server.
The messages/events log (<ul id="websocket_events">): displays all WebSocket-related events (e.g., connection status, received messages).

The JavaScript logic

Initializing the WebSocket connection

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:

onopen (the connection is established)

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

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!".

onmessage (a message is received)

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

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.

onclose (the connection is closed)

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

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.

onerror (an error is occurred)

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

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

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

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

append = function (text) {
    document
        .getElementById("websocket_events")
        .insertAdjacentHTML("beforeend", "<li>" + text + ";</li>");
};

This utility function adds WebSocket events and messages to the <ul> list (id="websocket_events").
insertAdjacentHTML("beforeend", "<li>" + text + ";</li>"): inserts the given text as a new list item (<li>) at the end of the list.

Styling with PicoCSS

<link
    rel="stylesheet"
    href="https://cdn.jsdelivr.net/npm/@picocss/pico@2/css/pico.min.css"
/>

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

When the page loads, the browser establishes a WebSocket connection with the server.
Upon successful connection, a message is logged saying, "🚀 Connected to WebSocket!".
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!

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