WebSocket server and client in Deno

Introduction

WebSocket lets your app talk to a server in real-time. It keeps the connection open so messages can flow back and forth instantly.

You want a chat app where messages appear instantly.

You need live updates like stock prices or sports scores.

You want a game where players see each other's moves right away.

You want to show notifications as soon as something happens.

You want to sync data between users without refreshing the page.

Syntax

Deno

import { serve } from "https://deno.land/std@0.177.0/http/server.ts";

serve(async (req) => {
  const { socket, response } = Deno.upgradeWebSocket(req);

  socket.onopen = () => {
    socket.send("Hello from server!");
  };

  socket.onmessage = (event) => {
    socket.send(`You said: ${event.data}`);
  };

  socket.onclose = () => {
    console.log("WebSocket closed");
  };

  return response;
}, { port: 8080 });

Use Deno.upgradeWebSocket(req) to turn an HTTP request into a WebSocket connection.

Handle events like onopen, onmessage, and onclose on the socket.

Examples

This is a simple WebSocket client connecting to a server at ws://localhost:8080.

Deno

const ws = new WebSocket("ws://localhost:8080");

ws.onopen = () => {
  ws.send("Hi server!");
};

ws.onmessage = (event) => {
  console.log("Message from server:", event.data);
};

Server replies with "pong" when it receives "ping" from the client.

Deno

socket.onmessage = (event) => {
  if (event.data === "ping") {
    socket.send("pong");
  }
};

Sample Program

This Deno program starts a WebSocket server on port 8080. It welcomes clients, echoes messages back with a prefix, and logs connection events.

Deno

import { serve } from "https://deno.land/std@0.177.0/http/server.ts";

console.log("WebSocket server running on ws://localhost:8080");

serve(async (req) => {
  if (req.headers.get("upgrade") !== "websocket") {
    return new Response("Not a websocket request", { status: 400 });
  }

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

  socket.onopen = () => {
    console.log("Client connected");
    socket.send("Welcome to the WebSocket server!");
  };

  socket.onmessage = (event) => {
    console.log("Received from client:", event.data);
    socket.send(`Server says: You sent '${event.data}'`);
  };

  socket.onclose = () => {
    console.log("Client disconnected");
  };

  socket.onerror = (e) => {
    console.error("WebSocket error:", e);
  };

  return response;
}, { port: 8080 });

Important Notes

WebSocket connections start as HTTP requests and then upgrade to a persistent connection.

Always check if the request wants to upgrade to WebSocket before upgrading.

Use console logs to see connection and message events during development.

Summary

WebSocket keeps a live connection open for instant two-way communication.

Deno uses Deno.upgradeWebSocket() to create WebSocket servers.

Clients use the WebSocket API to connect and talk to the server.

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