Why Deno's runtime features go beyond Node.js - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import a module in Deno using a URL.

Deno

import { serve } from "[1]";

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

A"https://deno.land/std/http/server.ts"

B"fs"

C"http"

D"./server.js"

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to import from local paths without extensions

Using Node.js style imports

Deno allows importing modules directly from URLs, unlike Node.js which uses local or npm modules.

2fill in blank

medium

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

Deno

const server = serve({ port: [1] });

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

A"8080"

Bport

C8080

D80

Attempts:

3 left

💡 Hint

Common Mistakes

Using port as a string

Using undefined variables

The port number must be a number, not a string or variable name.

3fill in blank

hard

Fix the error in the permission flag to allow file reading in Deno.

Deno

deno run --allow-[1] script.ts

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

Awrite

Bnet

Cenv

Dread

Attempts:

3 left

💡 Hint

Common Mistakes

Using --allow-write instead of --allow-read

Using no permission flag

To read files, Deno requires the --allow-read permission flag.

4fill in blank

hard

Fill both blanks to correctly use Deno's built-in test runner.

Deno

Deno.[1]("example test", () => {
  if (1 + 1 !== 2) throw new Error("Fail");
});

Deno.[2]();

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

Atest

BrunTests

Crun

Dstart

Attempts:

3 left

💡 Hint

Common Mistakes

Using Deno.run which runs subprocesses

Using wrong method names

Deno.test defines a test, and Deno.runTests executes tests programmatically.

5fill in blank

hard

Fill all three blanks to create a secure Deno script that reads a file and logs its content.

Deno

const data = await Deno.[1]("example.txt", { [2]: "utf-8" });
console.[3](data);

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

AreadTextFile

B"encoding"

Clog

Dprint

Attempts:

3 left

💡 Hint

Common Mistakes

Using readFile without decoding

Using console.print which doesn't exist

Deno.readTextFile reads a file as text, and console.log outputs it to the console.

Practice

1. Which of the following is a key security feature that Deno provides by default compared to Node.js?

easy

A. Deno automatically runs all scripts with full system access.

B. Deno requires explicit permission to access files, network, or environment.

C. Deno disables all network access permanently.

D. Deno uses a firewall to block unauthorized connections.

Solution

1. Step 1: Understand Deno's permission model

   Deno requires you to explicitly grant permissions for file, network, or environment access to improve security.

2. Step 2: Compare with Node.js default behavior

   Node.js scripts run with full access by default, unlike Deno's explicit permission system.

3. Final Answer:

   Deno requires explicit permission to access files, network, or environment. -> Option B

4. Quick Check:

   Explicit permissions = Deno security [OK]

Hint: Remember: Deno asks permission before access [OK]

Common Mistakes:

• Thinking Deno runs scripts with full access by default
• Confusing Deno's permission system with firewall rules
• Assuming network access is always blocked

2. Which syntax correctly runs a Deno script with network access permission?

easy

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

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

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

D. deno run --network script.ts

Solution

1. Step 1: Recall Deno permission flags

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

2. Step 2: Identify correct flag syntax

   The correct flag is exactly --allow-net; other options are invalid.

3. Final Answer:

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

4. Quick Check:

   Correct flag is --allow-net [OK]

Hint: Use --allow-net to enable network access [OK]

Common Mistakes:

• Using incorrect flag names like --net-allow
• Omitting the --allow-net flag when network access is needed
• Confusing permission flags with environment variables

3. What will be the output of this Deno script run with deno run script.ts if the script tries to read a file without permissions?

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

medium

A. PermissionDenied: read access to "secret.txt" is not allowed

B. SyntaxError: await used outside async function

C. FileNotFoundError: secret.txt does not exist

D. Contents of secret.txt printed to console

Solution

1. Step 1: Understand Deno's default permissions

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

2. Step 2: Predict error when reading file without permission

   Trying to read a file without permission causes a PermissionDenied error.

3. Final Answer:

   PermissionDenied: read access to "secret.txt" is not allowed -> Option A

4. Quick Check:

   Missing --allow-read causes PermissionDenied [OK]

Hint: No --allow-read means file read fails [OK]

Common Mistakes:

• Assuming file reads succeed without permission
• Confusing file not found with permission error
• Ignoring that await is valid in top-level for Deno

4. Identify the error in this Deno command and fix it:
deno run --allow-net --allow-read script.ts
Script still fails to access environment variables.

medium

A. No fix needed; environment variables are always accessible

B. Remove --allow-net flag to fix environment access

C. Use --allow-env instead of --allow-read

D. Add --allow-env flag to grant environment access

Solution

1. Step 1: Check permissions granted by command

   The command grants network and read permissions but not environment access.

2. Step 2: Identify missing permission for environment variables

   Accessing environment variables requires --allow-env flag.

3. Final Answer:

   Add --allow-env flag to grant environment access -> Option D

4. Quick Check:

   Missing --allow-env blocks env access [OK]

Hint: Add --allow-env to access environment variables [OK]

Common Mistakes:

• Assuming --allow-read covers environment access
• Removing unrelated flags instead of adding missing ones
• Believing environment variables are accessible without permission

5. You want to run a Deno script that uses TypeScript, formats code, lints, and accesses the network securely. Which combination of commands and features best uses Deno's runtime advantages?

hard

A. Use deno run --allow-net script.ts, then run deno fmt and deno lint commands

B. Use deno run --allow-net script.ts and rely on Node.js tools for formatting and linting

C. Run with deno run --allow-net script.ts, then manually install formatter and linter

D. Run script with deno run script.ts without permissions and use external tools for formatting

Solution

1. Step 1: Recognize Deno's built-in tools

   Deno includes built-in formatter (deno fmt) and linter (deno lint) without extra installs.

2. Step 2: Combine permissions and tools properly

   Run script with --allow-net for network, then use deno fmt and deno lint commands for formatting and linting.

3. Final Answer:

   Use deno run --allow-net script.ts, then run deno fmt and deno lint commands -> Option A

4. Quick Check:

   Deno built-in tools + explicit permissions = best practice [OK]

Hint: Use deno fmt and lint with permissions for best workflow [OK]

Common Mistakes:

• Assuming formatter and linter need separate installs
• Running without permissions and expecting network access
• Using Node.js tools instead of Deno's built-in ones