WebSocket server and client in Deno - Mini Project: Build & Apply

WebSocket Server and Client with Deno

📖 Scenario: You want to create a simple chat system where a server can send messages to connected clients in real-time using WebSockets. This is like a walkie-talkie where messages are instantly shared.

🎯 Goal: Build a WebSocket server that accepts client connections and a client that connects to the server and sends a message. The server should broadcast received messages to all connected clients.

📋 What You'll Learn

Create a WebSocket server that listens on port 8080

Create a client that connects to the WebSocket server

Send a message from the client to the server

Broadcast messages from the server to all connected clients

💡 Why This Matters

🌍 Real World

WebSockets are used in chat apps, live notifications, multiplayer games, and real-time dashboards where instant communication is needed.

💼 Career

Understanding WebSocket servers and clients is important for backend and frontend developers working on real-time web applications.

1

Create a WebSocket server listening on port 8080

Write code to create a WebSocket server using Deno's standard library that listens on port 8080. Store the server listener in a variable called server.

Deno

# Your code here to create a WebSocket server listening on port 8080

Hint

Use serve from Deno's standard HTTP server module and specify port: 8080.

2

Accept WebSocket connections and store clients

Add code to accept incoming HTTP requests from server. Upgrade requests to WebSocket connections and store each connected WebSocket in a clients set.

Deno

import { serve } from "https://deno.land/std@0.203.0/http/server.ts";

const server = serve({ port: 8080 });

// Your code here to accept WebSocket connections and store clients

Hint

Use Deno.upgradeWebSocket(req) to upgrade HTTP requests to WebSocket connections. Store sockets in a Set called clients.

3

Broadcast messages received from clients to all clients

Inside the WebSocket connection handler, add an onmessage event that sends the received message data to all connected clients in the clients set.

Deno

import { serve } from "https://deno.land/std@0.203.0/http/server.ts";

const server = serve({ port: 8080 });

const clients = new Set();

for await (const req of server) {
  const { socket, response } = Deno.upgradeWebSocket(req);
  clients.add(socket);
  socket.onclose = () => {
    clients.delete(socket);
  };
  // Your code here to broadcast messages to all clients
  req.respond(response);
}

Hint

Use socket.onmessage to listen for messages and loop over clients to send the message to each.

4

Create a WebSocket client that connects and sends a message

Write code to create a WebSocket client that connects to ws://localhost:8080. After connection, send the message "Hello from client".

Deno

import { serve } from "https://deno.land/std@0.203.0/http/server.ts";

const server = serve({ port: 8080 });

const clients = new Set();

for await (const req of server) {
  const { socket, response } = Deno.upgradeWebSocket(req);
  clients.add(socket);
  socket.onmessage = (event) => {
    for (const client of clients) {
      client.send(event.data);
    }
  };
  socket.onclose = () => {
    clients.delete(socket);
  };
  req.respond(response);
}

// Your code here to create a WebSocket client and send a message

Hint

Create a WebSocket object with the server URL and send a message inside onopen.

Practice

1. What is the main purpose of using WebSocket in a Deno application?

easy

A. To keep a live connection open for instant two-way communication

B. To send emails from the server

C. To serve static HTML files

D. To run background tasks without user interaction

Solution

1. Step 1: Understand WebSocket functionality

   WebSocket allows a continuous open connection between client and server for real-time data exchange.

2. Step 2: Compare with other options

   Sending emails, serving static files, or running background tasks are unrelated to WebSocket's purpose.

3. Final Answer:

   To keep a live connection open for instant two-way communication -> Option A

4. Quick Check:

   WebSocket = live two-way connection [OK]

Hint: WebSocket means live chat-like connection [OK]

Common Mistakes:

• Confusing WebSocket with HTTP request-response
• Thinking WebSocket sends emails
• Assuming WebSocket serves files

2. Which of the following is the correct way to upgrade an HTTP request to a WebSocket connection in Deno?

easy

A. const { socket, response } = WebSocket.upgrade(request);

B. const socket = new WebSocket(request);

C. const ws = new WebSocketServer(request);

D. const { socket, response } = Deno.upgradeWebSocket(request);

Solution

1. Step 1: Recall Deno WebSocket upgrade syntax

   Deno uses Deno.upgradeWebSocket(request) to upgrade HTTP requests to WebSocket connections.

2. Step 2: Check other options for correctness

   Options B, C, and D use incorrect constructors or methods not available in Deno's standard library.

3. Final Answer:

   const { socket, response } = Deno.upgradeWebSocket(request); -> Option D

4. Quick Check:

   Deno.upgradeWebSocket() = correct upgrade method [OK]

Hint: Use Deno.upgradeWebSocket() to upgrade requests [OK]

Common Mistakes:

• Using new WebSocket() on server side
• Calling non-existent WebSocketServer constructor
• Mixing client WebSocket API with server upgrade

3. Given this Deno WebSocket server snippet, what will the server send back when a client sends 'hello'?

const { socket, response } = Deno.upgradeWebSocket(request);
socket.onmessage = (e) => {
  socket.send(`Received: ${e.data}`);
};

medium

A. "hello"

B. "e.data"

C. "Received: hello"

D. No response sent

Solution

1. Step 1: Analyze the onmessage handler

   When a message arrives, the server sends back a string with 'Received: ' plus the message content.

2. Step 2: Substitute the client message

   Client sends 'hello', so server sends 'Received: hello'.

3. Final Answer:

   "Received: hello" -> Option C

4. Quick Check:

   Server echoes with prefix 'Received:' [OK]

Hint: Server sends back 'Received: ' plus client message [OK]

Common Mistakes:

• Thinking server echoes raw message without prefix
• Confusing e.data as string literal
• Assuming no response is sent

4. What is wrong with this Deno WebSocket server code?

const { socket, response } = Deno.upgradeWebSocket(request);
socket.onmessage = (e) => {
  socket.send(e.data.toUpperCase());
};
return response;

medium

A. No error, code is correct

B. Not returning the response object

C. Missing await before Deno.upgradeWebSocket

D. Not handling socket.onopen event

Solution

1. Step 1: Check upgradeWebSocket usage

   Deno.upgradeWebSocket is synchronous and returns socket and response correctly.

2. Step 2: Verify response return and event handling

   The response is returned properly, and onopen event is optional, not required.

3. Final Answer:

   No error, code is correct -> Option A

4. Quick Check:

   Code follows correct WebSocket server pattern [OK]

Hint: upgradeWebSocket is synchronous; return response is required [OK]

Common Mistakes:

• Adding unnecessary await to upgradeWebSocket
• Forgetting to return response
• Thinking onopen event is mandatory

5. You want to build a Deno WebSocket server that broadcasts any received message to all connected clients. Which approach correctly manages multiple clients?

hard

A. Use a single socket variable and overwrite it on each connection

B. Store all sockets in an array and loop to send messages to each socket

C. Create a new WebSocket server for each client connection

D. Send messages only to the last connected client

Solution

1. Step 1: Understand broadcasting concept

   Broadcasting means sending a message to all connected clients, so all sockets must be stored.

2. Step 2: Evaluate options for managing clients

   Store all sockets in an array and loop to send messages to each socket stores sockets in an array and loops to send messages, which is correct. Options A, B, and D do not support broadcasting properly.

3. Final Answer:

   Store all sockets in an array and loop to send messages to each socket -> Option B

4. Quick Check:

   Broadcast = send to all sockets stored [OK]

Hint: Keep all sockets in a list to broadcast messages [OK]

Common Mistakes:

• Overwriting socket variable losing previous clients
• Creating multiple servers instead of one
• Sending messages only to one client