Why production Deno needs careful architecture - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import a module in Deno.

Deno

import { [1] } from "https://deno.land/std@0.177.0/fs/mod.ts";

Drag options to blanks, or click blank then click option'

AreadTextFile

BreadFileStr

CreadDir

DreadFileSync

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a function that reads file contents instead of directory listing.

Using a function not available in the imported module.

The readDir function is imported from Deno's standard library to read directory contents.

2fill in blank

medium

Complete the code to start a simple HTTP server in Deno.

Deno

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

serve(() => new Response("Hello, Deno!"), { port: [1] });

Drag options to blanks, or click blank then click option'

A8080

B5000

C80

D3000

Attempts:

3 left

💡 Hint

Common Mistakes

Using port 80 without proper permissions.

Choosing a port number that might be blocked or reserved.

Port 8080 is commonly used for development servers and is a good choice here.

3fill in blank

hard

Fix the error in the code to correctly read environment variables in Deno.

Deno

const dbPassword = Deno.[1].get("DB_PASSWORD");

Drag options to blanks, or click blank then click option'

Aenvironment

Benv

Cenvs

DgetEnv

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect property names like 'envs' or 'environment'.

Trying to call a method instead of accessing a property.

Deno provides environment variables through Deno.env. The get method retrieves the variable value.

4fill in blank

hard

Fill both blanks to create a secure file read function that handles errors in Deno.

Deno

async function readConfig(path) {
  try {
    const data = await Deno.[1](path, { [2]: "utf8" });
    return data;
  } catch {
    return null;
  }
}

Drag options to blanks, or click blank then click option'

AreadTextFile

BreadFile

Cencoding

Dmode

Attempts:

3 left

💡 Hint

Common Mistakes

Using readFile which returns bytes, not text.

Using incorrect option keys like 'mode' instead of 'encoding'.

readTextFile reads a file as text, and encoding specifies the text encoding.

5fill in blank

hard

Fill all three blanks to create a Deno server that logs requests and responds with JSON.

Deno

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

serve(async (req) => {
  console.[1]("Request received");
  const data = { message: "Hello from Deno" };
  return new Response(JSON.[2](data), { headers: { "Content-Type": [3] } });
});

Drag options to blanks, or click blank then click option'

Alog

Bstringify

C"application/json"

Derror

Attempts:

3 left

💡 Hint

Common Mistakes

Using console.error instead of console.log.

Forgetting to stringify the JSON object.

Setting wrong content type header.

console.log prints messages, JSON.stringify converts objects to JSON strings, and the content type must be "application/json".

Practice

1. Why is it important to plan permissions carefully when deploying a Deno app to production?

easy

A. Because Deno restricts access to files, network, and environment by default for security.

B. Because Deno automatically grants all permissions to any script.

C. Because permissions do not affect Deno apps in production.

D. Because permissions only matter during development, not in production.

Solution

1. Step 1: Understand Deno's default security model

   Deno runs scripts with no permissions by default to protect your system.

2. Step 2: Recognize the need for explicit permission granting in production

   In production, you must carefully grant only needed permissions to avoid security risks.

3. Final Answer:

   Because Deno restricts access to files, network, and environment by default for security. -> Option A

4. Quick Check:

   Default security = careful permission planning [OK]

Hint: Remember: Deno locks down permissions by default [OK]

Common Mistakes:

• Assuming Deno grants all permissions automatically
• Ignoring permissions in production
• Thinking permissions only matter during development

2. Which of the following is the correct way to run a Deno script with network access permission?

easy

A. deno run --allownetwork script.ts

B. deno run --net-allow script.ts

C. deno run --enable-network script.ts

D. deno run --allow-net script.ts

Solution

1. Step 1: Recall Deno's permission flags syntax

   Deno uses flags like --allow-net to grant network access.

2. Step 2: Identify the correct flag from options

   Only --allow-net matches Deno's official permission flag syntax.

3. Final Answer:

   deno run --allow-net script.ts -> Option D

4. Quick Check:

   Correct flag syntax = --allow-net [OK]

Hint: Look for exact permission flag syntax with dashes [OK]

Common Mistakes:

• Using incorrect flag names like --net-allow
• Combining words without dashes
• Assuming permission flags are optional

3. Consider this Deno code snippet:

try {
  const data = await Deno.readTextFile('config.json');
  console.log('Config loaded:', data);
} catch (error) {
  console.error('Failed to load config:', error.message);
}

What will happen if the file config.json does not exist and the script is run with deno run --allow-read script.ts?

medium

A. The catch block will run and print an error message.

B. The script will crash with an uncaught error.

C. The script will print 'Config loaded:' with empty data.

D. The script will silently ignore the error and continue.

Solution

1. Step 1: Understand try-catch behavior in async file reading

   If the file is missing, Deno.readTextFile throws an error caught by catch.

2. Step 2: Analyze the catch block's effect

   The catch block logs 'Failed to load config:' plus the error message.

3. Final Answer:

   The catch block will run and print an error message. -> Option A

4. Quick Check:

   Missing file triggers catch block [OK]

Hint: Missing file triggers catch block error handling [OK]

Common Mistakes:

• Expecting the script to crash without catch
• Assuming empty string is returned for missing file
• Thinking error is ignored silently

4. You have this Deno script:

const data = await Deno.readTextFile('data.txt');
console.log(data);

When running deno run script.ts without any flags, what error will you get?

medium

A. SyntaxError: Unexpected token

B. PermissionDenied: read access to "data.txt" is not allowed

C. ReferenceError: Deno is not defined

D. No error, script runs fine

Solution

1. Step 1: Recall Deno's default permission restrictions

   Deno denies file read access unless --allow-read is given.

2. Step 2: Identify the error from missing permission

   Running without --allow-read causes a PermissionDenied error.

3. Final Answer:

   PermissionDenied: read access to "data.txt" is not allowed -> Option B

4. Quick Check:

   Missing --allow-read causes PermissionDenied [OK]

Hint: No --allow-read means PermissionDenied error [OK]

Common Mistakes:

• Expecting syntax or reference errors
• Assuming script runs without permissions
• Confusing permission errors with runtime errors

5. You want to build a Deno app that reads a config file, fetches data from an API, and writes logs to a file. Which architecture approach best ensures security and maintainability in production?

hard

A. Grant all permissions globally and write all code in one file.

B. Run the app without permissions and rely on user input for data.

C. Use modular code, grant only needed permissions per module, and handle errors properly.

D. Avoid error handling to keep code simple and grant all permissions at runtime.

Solution

1. Step 1: Consider security by limiting permissions

   Grant only the permissions each module needs to reduce risk.

2. Step 2: Organize code modularly and handle errors

   Modular code improves maintainability; error handling improves reliability.

3. Final Answer:

   Use modular code, grant only needed permissions per module, and handle errors properly. -> Option C

4. Quick Check:

   Modular + limited permissions + error handling = best practice [OK]

Hint: Modular code + least permissions + error handling wins [OK]

Common Mistakes:

• Granting all permissions globally
• Ignoring error handling
• Putting all code in one file