Posted onDec 1, 2024

WebSocket Client with JavaScript

#websocket #javascript #webdev #programming

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 theindex.html file when a request is made to/. Theindex.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 thefetch method of the server explained in "WebSocket with JavaScript and Bun" is implemented this code:

if(url.pathname==="/")returnnewResponse(Bun.file("./index.html"));

This means that when a browser request is made tohttp://localhost:8080/, the content of theindex.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>letecho_service;append=function(text){document.getElementById("websocket_events").insertAdjacentHTML("beforeend","<li>"+text+";</li>");};window.onload=function(){echo_service=newWebSocket("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");};};functionsendMessage(event){console.log(event);letmessage=document.getElementById("message").value;echo_service.send(message);}</script><linkrel="stylesheet"href="https://cdn.jsdelivr.net/npm/@picocss/pico@2/css/pico.min.css"/></head><body><mainclass="container">            Message:<inputvalue="Hello!"type="text"id="message"/><inputtype="button"value="Submit"onclick="sendMessage(event)"/><br/><ulid="websocket_events"></ul></main></body></html>

Explaining the client code

This code creates a simpleWebSocket 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><mainclass="container">            Message:<inputvalue="Hello!"type="text"id="message"/><inputtype="button"value="Submit"onclick="sendMessage(event)"/><br/><ulid="websocket_events"></ul></main></body></html>

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

The JavaScript logic

Initializing the WebSocket connection

window.onload=function(){echo_service=newWebSocket("ws://127.0.0.1:8080/chat");...};

WebSocket("ws://127.0.0.1:8080/chat"): creates a new WebSocket connection to the server at127.0.0.1 on port8080, specifically the/chat endpoint.
The variableecho_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!");};

Theonopen 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);};

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

onclose (the connection is closed)

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

Theonclose 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");};

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

Sending messages to the server

functionsendMessage(event){letmessage=document.getElementById("message").value;echo_service.send(message);}

ThesendMessage 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

<linkrel="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. Inthe 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!