What is Deno - Practice Questions & Exercises

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is the main purpose of Deno?

Deno is a modern runtime for JavaScript and TypeScript. What is its main goal?

A To provide a secure and simple runtime for JavaScript and TypeScript with built-in utilities.

B To replace Python as the primary language for backend development.

C To serve as a browser extension for running JavaScript faster.

D To act as a database management system for web applications.

💡 Hint

Think about what Deno offers beyond just running code.

Deno is designed to run JavaScript and TypeScript securely with built-in tools like a package manager and formatter.

❓ component_behavior

intermediate

How does Deno handle permissions?

Deno runs scripts with strict permissions. What happens if a script tries to read a file without permission?

Deno

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

A The script ignores the read request and continues silently.

B The script throws a permission error and stops execution.

C The script automatically requests permission from the user.

D The script reads the file without any issues.

💡 Hint

Deno is designed to be secure by default.

Deno requires explicit permission to access files. Without permission, it throws an error to protect the system.

📝 Syntax

advanced

What is the correct way to run a TypeScript file with Deno?

You have a file named app.ts. Which command runs it correctly with Deno?

A node app.ts

B deno execute app.ts

C tsc app.ts && deno run app.js

D deno run app.ts

💡 Hint

Deno supports TypeScript natively without compiling first.

Deno can run TypeScript files directly using deno run. No separate compilation is needed.

🔧 Debug

advanced

Why does this Deno script fail to run?

Consider this script saved as read.ts:

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

Running deno run read.ts causes an error. Why?

A Because the script needs the <code>--allow-read</code> flag to access files.

B Because <code>await</code> cannot be used outside a function in Deno.

C Because the file <code>info.txt</code> does not exist.

D Because Deno does not support reading text files.

💡 Hint

Think about Deno's security model and permissions.

Deno requires explicit permission flags like --allow-read to read files. Without it, the script fails.

❓ state_output

expert

What is the output of this Deno script?

Given the following script counter.ts:

let count = 0;

function increment() {
  count += 1;
  return count;
}

console.log(increment());
console.log(increment());
console.log(count);

What will be printed when running deno run counter.ts?

A

1
1
2

B

0
1
2

C

1
2
2

D

1
2
3

💡 Hint

Track how the variable count changes after each function call.

The function increments count by 1 each time and returns it. So the outputs are 1, then 2, then the final count is 2.

Practice

1. What is Deno primarily used for?

easy

A. Running JavaScript and TypeScript outside the browser

B. Creating mobile apps

C. Designing websites with HTML and CSS

D. Managing databases

Solution

1. Step 1: Understand Deno's purpose

   Deno is a runtime environment that allows running JavaScript and TypeScript code outside of web browsers.

2. Step 2: Compare options with Deno's use

   Options A, B, and D describe other technologies or tasks unrelated to Deno's main function.

3. Final Answer:

   Running JavaScript and TypeScript outside the browser -> Option A

4. Quick Check:

   Deno runs JS/TS outside browser = C [OK]

Hint: Deno runs JS/TS outside browser, not for apps or design [OK]

Common Mistakes:

• Confusing Deno with frontend frameworks
• Thinking Deno manages databases
• Assuming Deno is for mobile app development

2. Which command correctly runs a Deno script named app.ts?

easy

A. deno execute app.ts

B. deno start app.ts

C. node app.ts

D. deno run app.ts

Solution

1. Step 1: Recall Deno's run command

   Deno uses the command deno run to execute scripts.

2. Step 2: Check other options

   deno execute app.ts uses 'execute' which is invalid, deno start app.ts uses 'start' which is not a Deno command, node app.ts uses 'node' which is for Node.js.

3. Final Answer:

   deno run app.ts -> Option D

4. Quick Check:

   Run scripts with 'deno run' = D [OK]

Hint: Use 'deno run' to execute scripts, not 'execute' or 'start' [OK]

Common Mistakes:

• Using 'node' instead of 'deno run'
• Typing 'deno execute' instead of 'deno run'
• Trying 'deno start' which is invalid

3. What will happen if you run deno run --allow-net app.ts?

medium

A. The script runs with permission to access the network

B. The script runs without any permissions

C. The script will fail due to missing permissions

D. The script runs with permission to access the file system

Solution

1. Step 1: Understand the permission flag

   The flag --allow-net grants the script permission to access network resources.

2. Step 2: Analyze the command effect

   Since the flag is present, the script can use network features. Other permissions like file system access are not granted.

3. Final Answer:

   The script runs with permission to access the network -> Option A

4. Quick Check:

   --allow-net grants network access = A [OK]

Hint: Flags like --allow-net control permissions; here network is allowed [OK]

Common Mistakes:

• Assuming no permissions are given without --allow-all
• Confusing network permission with file system permission
• Thinking script fails without --allow-net

4. Identify the error in this command: deno run --allow-read app.ts when app.ts tries to access the network.

medium

A. No error, network access is allowed by default

B. Error because --allow-read does not allow network access

C. Error because --allow-read is an invalid flag

D. No error, script runs with all permissions

Solution

1. Step 1: Understand permission flags

   The flag --allow-read only allows file system read access, not network access.

2. Step 2: Analyze script behavior

   If app.ts tries to access the network without --allow-net, it will cause a permission error.

3. Final Answer:

   Error because --allow-read does not allow network access -> Option B

4. Quick Check:

   --allow-read ≠ network permission = B [OK]

Hint: Network needs --allow-net; --allow-read is only for files [OK]

Common Mistakes:

• Assuming --allow-read grants network access
• Thinking all permissions are granted by default
• Using invalid flags

5. You want to run a Deno script server.ts that reads files and accesses the network securely. Which command should you use?

hard

A. deno run server.ts

B. deno run --allow-all server.ts

C. deno run --allow-net --allow-read server.ts

D. deno run --allow-write server.ts

Solution

1. Step 1: Identify required permissions

   The script needs to read files and access the network, so it requires --allow-read and --allow-net.

2. Step 2: Evaluate options for security

   deno run --allow-net --allow-read server.ts grants only needed permissions. deno run --allow-all server.ts grants all permissions, which is less secure. deno run server.ts grants none. deno run --allow-write server.ts grants write access, which is unnecessary.

3. Final Answer:

   deno run --allow-net --allow-read server.ts -> Option C

4. Quick Check:

   Grant only needed permissions for security = A [OK]

Hint: Grant only needed permissions, avoid --allow-all for safety [OK]

Common Mistakes:

• Using --allow-all which is less secure
• Forgetting to add --allow-net for network access
• Adding unnecessary permissions like --allow-write