Running your first Deno program - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple Deno script

What will be the output when running this Deno script?

Deno

console.log("Hello from Deno!");

A Hello from Deno!

B undefined

C SyntaxError

D ReferenceError

💡 Hint

Think about what console.log does in JavaScript and Deno.

The console.log function prints the string to the console. So the output is exactly the string inside the quotes.

📝 Syntax

intermediate

Identify the syntax error in a Deno script

Which option contains a syntax error when running in Deno?

Deno

console.log("Welcome to Deno")
console.log('Enjoy coding!')

A

console.log("Welcome to Deno")
console.log('Enjoy coding!')

B

console.log("Welcome to Deno";
console.log('Enjoy coding!')

C

console.log("Welcome to Deno");
console.log('Enjoy coding!')

D

console.log("Welcome to Deno")
console.log('Enjoy coding!');

💡 Hint

Look carefully at the punctuation marks in each line.

Option B has a missing closing parenthesis before the semicolon, causing a syntax error.

❓ component_behavior

advanced

Behavior of Deno script with async function

What will be the output of this Deno script?

Deno

async function greet() {
  return "Hello, Deno!";
}

greet().then(console.log);

A undefined

B Promise { 'Hello, Deno!' }

C SyntaxError

D Hello, Deno!

💡 Hint

Remember that .then() waits for the promise to resolve and passes the result.

The async function returns a promise that resolves to the string. The .then(console.log) prints the resolved value.

🔧 Debug

advanced

Identify the runtime error in this Deno script

Which option will cause a runtime error when running this Deno script?

Deno

console.log(message);
const message = "Deno is awesome!";

A ReferenceError: Cannot access 'message' before initialization

B undefined

C SyntaxError

D Deno is awesome!

💡 Hint

Think about variable hoisting and when variables are available.

The variable 'message' is declared with const after it is used, causing a ReferenceError at runtime.

🧠 Conceptual

expert

Understanding Deno permissions

What happens if you run a Deno script that tries to read a file without the proper permission flag?

Deno

import { readTextFile } from "https://deno.land/std/fs/mod.ts";

const content = await readTextFile("./data.txt");
console.log(content);

A The script logs undefined and continues

B The file is read successfully without any warnings

C Deno throws a PermissionDenied error and stops execution

D The script crashes with a SyntaxError

💡 Hint

Deno requires explicit permission for file system access.

Deno enforces security by requiring permission flags like --allow-read to access files. Without it, it throws a PermissionDenied error.

Practice

1. What command do you use to run a JavaScript or TypeScript file with Deno?

easy

A. node filename.js

B. deno run filename.ts

C. npm start filename.ts

D. deno start filename.js

Solution

1. Step 1: Understand the Deno run command

   Deno uses the command deno run followed by the file name to execute scripts.

2. Step 2: Identify the correct command for running a file

   The correct syntax is deno run filename.ts or deno run filename.js. Other options are for Node.js or incorrect commands.

3. Final Answer:

   deno run filename.ts -> Option B

4. Quick Check:

   Deno runs scripts with 'deno run' [OK]

Hint: Use 'deno run' plus your file name to run scripts [OK]

Common Mistakes:

• Using 'node' instead of 'deno run'
• Typing 'deno start' which is invalid
• Using 'npm start' which is for Node.js projects

2. Which of the following is the correct syntax to print 'Hello, Deno!' to the console in a Deno program?

easy

A. console.print('Hello, Deno!')

B. print('Hello, Deno!')

C. console.log('Hello, Deno!')

D. echo('Hello, Deno!')

Solution

1. Step 1: Recall JavaScript console output method

   In JavaScript and TypeScript, the standard way to print to the console is using console.log().

2. Step 2: Identify the correct function

   Options like print() or echo() are not valid in JavaScript or Deno. console.print() is not a function.

3. Final Answer:

   console.log('Hello, Deno!') -> Option C

4. Quick Check:

   Use console.log() to print text [OK]

Hint: Use console.log() to print messages in Deno [OK]

Common Mistakes:

• Using print() which is not defined in JavaScript
• Trying console.print() which does not exist
• Using echo() which is from shell scripting

3. What will be the output when you run this Deno program?

console.log('Welcome to Deno!');

medium

A. Welcome to Deno!

B. console.log('Welcome to Deno!');

C. SyntaxError

D. No output

Solution

1. Step 1: Understand console.log behavior

   The console.log() function prints the string inside the parentheses to the console.

2. Step 2: Predict the output

   The string 'Welcome to Deno!' will be printed exactly as is, without quotes.

3. Final Answer:

   Welcome to Deno! -> Option A

4. Quick Check:

   console.log prints the string content [OK]

Hint: console.log prints text without quotes [OK]

Common Mistakes:

• Expecting the code line itself as output
• Thinking it causes a syntax error
• Assuming no output without running

4. You wrote this Deno program:

console.log('Hello Deno!')

But when you run deno run hello.ts, you get an error. What is the most likely cause?

medium

A. You must run with 'deno start' instead

B. You need to add a semicolon at the end

C. Deno does not support console.log

D. You forgot to save the file before running

Solution

1. Step 1: Check common causes of errors when running Deno

   If the file is not saved, Deno cannot find the latest code to run, causing errors.

2. Step 2: Evaluate other options

   Semicolons are optional in JavaScript/TypeScript, console.log is supported, and 'deno start' is not a valid command.

3. Final Answer:

   You forgot to save the file before running -> Option D

4. Quick Check:

   Unsaved files cause run errors [OK]

Hint: Always save your file before running with Deno [OK]

Common Mistakes:

• Thinking semicolons are mandatory
• Believing console.log is unsupported
• Using wrong Deno commands like 'deno start'

5. You want to run a Deno program named greet.ts that prints a greeting. Which command will correctly run the program if your terminal is in the same folder as the file?

medium

A. deno run ./greet.ts

B. deno start greet.ts

C. deno run ../greet.ts

D. deno rungreet.ts

Solution

1. Step 1: Understand file path usage in Deno run

   Deno requires the file path to be correct. Using ./greet.ts explicitly points to the current directory.

2. Step 2: Evaluate other options

   deno rungreet.ts is invalid because there's no space after run. deno run ../greet.ts points to the parent directory, which is wrong here. deno start greet.ts is invalid.

3. Final Answer:

   deno run ./greet.ts -> Option A

4. Quick Check:

   Use './filename' to run files in current folder [OK]

Hint: Use './filename' to run files in current directory [OK]

Common Mistakes:

• Missing space after 'run' like 'deno rungreet.ts'
• Using '../' which points to wrong folder
• Trying 'deno start' which is invalid