WebSocket server and client in Deno - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a WebSocket in simple terms?

A WebSocket is like a two-way phone line between a web browser and a server. It lets them talk to each other instantly without waiting for each message to finish.

beginner

How do you create a WebSocket server in Deno?

You use Deno's standard library to listen for WebSocket connections on a port, then accept and handle messages from clients.

intermediate

What method do you use to accept a WebSocket connection in Deno's HTTP server?

You use the `Deno.upgradeWebSocket(request)` method to upgrade an HTTP request to a WebSocket connection.

beginner

How does a WebSocket client connect to a server in Deno?

The client uses the `WebSocket` constructor with the server URL, then listens for open, message, and close events to communicate.

beginner

Why is WebSocket better than regular HTTP for real-time apps?

WebSocket keeps the connection open, so messages go back and forth instantly without the delay of opening new connections each time.

Which Deno method upgrades an HTTP request to a WebSocket connection?

A Deno.upgradeWebSocket(request)

B Deno.createWebSocket()

C Deno.openSocket()

D Deno.connectWebSocket()

Deno.upgradeWebSocket(request) is the correct method to upgrade an HTTP request to a WebSocket connection.

What event should a WebSocket client listen to for receiving messages?

A message

B open

C connect

D send

The 'message' event is fired when the client receives data from the server.

In Deno, which module commonly provides WebSocket server utilities?

A std/http

B std/ws

C std/fs

D std/net

The std/ws module in Deno's standard library provides WebSocket utilities.

What is the main advantage of WebSocket over HTTP for communication?

A Uses less memory

B Requires no server

C Works without internet

D Supports two-way instant communication

WebSocket supports two-way instant communication by keeping the connection open.

Which URL scheme is used to connect to a WebSocket server?

A http://

B https://

C ws://

D ftp://

The 'ws://' scheme is used for WebSocket connections; 'wss://' is used for secure WebSocket.

Explain how to set up a basic WebSocket server in Deno and handle incoming messages.

Think about upgrading HTTP requests and event listeners.

Describe the steps a WebSocket client in Deno takes to connect and communicate with a server.

Focus on connection and event handling.

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