Deno REPL for quick testing - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of variable reassignment in Deno REPL

What will be the output after running these lines in the Deno REPL?

Deno

let count = 5;
count = count + 3;
console.log(count);

A 8

B 5

C Error: count is not defined

D undefined

💡 Hint

Remember that variables declared with let can be reassigned.

The variable 'count' starts at 5, then 3 is added, so the output is 8.

❓ component_behavior

intermediate

Behavior of top-level await in Deno REPL

What will the Deno REPL output after running this code snippet?

Deno

const result = await Promise.resolve('done');
console.log(result);

A SyntaxError: await is only valid in async function

B done

C Promise { 'done' }

D undefined

💡 Hint

Deno REPL supports top-level await by default.

Deno REPL allows top-level await, so the promise resolves and logs 'done'.

📝 Syntax

advanced

Identify the syntax error in Deno REPL code

Which option contains a syntax error when entered in the Deno REPL?

A const x = {a: 1, b: 2};

B function test() { return 5; }

C let y = [1, 2, 3;

D console.log('Hello Deno');

💡 Hint

Look for missing closing brackets or parentheses.

Option C is missing the closing bracket for the array, causing a syntax error.

🔧 Debug

advanced

Why does this Deno REPL code throw a ReferenceError?

Given this code in Deno REPL, why does it throw a ReferenceError?

Deno

console.log(value);
let value = 10;

A Because 'value' is used before declaration and let does not hoist initialization.

B Because console.log is not defined in Deno REPL.

C Because let variables cannot be logged.

D Because value is declared but never assigned.

💡 Hint

Think about variable hoisting and temporal dead zone.

Variables declared with let are not initialized before their declaration line, so accessing them earlier causes ReferenceError.

🧠 Conceptual

expert

Understanding module import behavior in Deno REPL

What happens when you run this import statement in the Deno REPL?

Deno

import { serve } from "https://deno.land/std@0.177.0/http/server.ts";
console.log(typeof serve);

A TypeError because serve is not exported from the module.

B SyntaxError because import statements are not allowed in Deno REPL.

C undefined because modules cannot be imported in REPL.

D "function" is logged because the module is imported and serve is a function.

💡 Hint

Deno REPL supports ES module imports with top-level await.

Deno REPL supports ES module imports, so serve is imported and is a function.

Practice

1. What is the main purpose of the deno repl command?

easy

A. To install Deno packages from the internet

B. To start an interactive environment for testing Deno code quickly

C. To compile Deno scripts into executable files

D. To create a new Deno project folder

Solution

1. Step 1: Understand what REPL means

   REPL stands for Read-Eval-Print Loop, an interactive shell to run code instantly.

2. Step 2: Match REPL purpose with Deno command

   The deno repl command opens this interactive shell for Deno code testing.

3. Final Answer:

   To start an interactive environment for testing Deno code quickly -> Option B

4. Quick Check:

   Deno REPL = quick interactive testing [OK]

Hint: REPL means interactive testing shell in Deno [OK]

Common Mistakes:

• Confusing REPL with compiling scripts
• Thinking REPL installs packages
• Assuming REPL creates project folders

2. Which of the following is the correct way to start the Deno REPL in your terminal?

easy

A. deno run repl

B. deno shell

C. deno start repl

D. deno repl

Solution

1. Step 1: Recall the exact command to start Deno REPL

   The official command is deno repl to launch the interactive shell.

2. Step 2: Check other options for correctness

   Options like deno run repl, deno start repl, and deno shell are invalid commands.

3. Final Answer:

   deno repl -> Option D

4. Quick Check:

   Start Deno REPL = deno repl [OK]

Hint: Use exactly 'deno repl' to start the REPL [OK]

Common Mistakes:

• Adding extra words like 'run' or 'start'
• Using 'deno shell' which is not a Deno command
• Confusing with Node.js REPL commands

3. What will be the output when you run the following code inside the Deno REPL?

const x = 5;
const y = 10;
x + y;

medium

A. 15

B. undefined

C. NaN

D. SyntaxError

Solution

1. Step 1: Evaluate the code line by line in REPL context

   Variables x and y are assigned 5 and 10 respectively.

2. Step 2: Calculate the expression x + y

   Adding 5 + 10 results in 15, which REPL will print as output.

3. Final Answer:

   15 -> Option A

4. Quick Check:

   5 + 10 = 15 [OK]

Hint: REPL prints last expression value automatically [OK]

Common Mistakes:

• Expecting no output or undefined
• Confusing with syntax errors
• Thinking variables are not stored in REPL

4. You typed the following in the Deno REPL but got an error:

let a = 10
let b = 20
console.log(a + b)

What is the likely cause of the error?

medium

A. Missing semicolons causing syntax error in REPL

B. Deno REPL does not support console.log

C. Variables must be declared with var, not let

D. You need to run the code in a file, not REPL

Solution

1. Step 1: Check syntax rules in Deno REPL

   Deno REPL requires semicolons or line breaks to separate statements properly.

2. Step 2: Identify missing semicolons causing error

   Missing semicolons after let a = 10 and let b = 20 cause syntax error in REPL.

3. Final Answer:

   Missing semicolons causing syntax error in REPL -> Option A

4. Quick Check:

   Semicolons needed in REPL lines [OK]

Hint: Add semicolons or press Enter twice in REPL [OK]

Common Mistakes:

• Assuming console.log is unsupported
• Thinking let is invalid in REPL
• Believing REPL can't run multi-line code

5. You want to quickly test a function that returns the square of a number in Deno REPL. Which sequence of commands will correctly define and test it?

hard

A. function square(n) {
return n * n
}
console.log(square(4));

B. function square(n) {
return n * n
}
square(4)

C. function square(n) {
return n * n;
}
square(4);

D. function square(n) {
return n * n;
}
console.log(square(4))

Solution

1. Step 1: Define the function with proper syntax and semicolons

   function square(n) {
     return n * n;
   }
   square(4);

   uses semicolons and braces correctly, which is good practice in REPL.

2. Step 2: Call the function as an expression to see output

   Calling square(4); as last line prints 16 automatically in REPL.

3. Final Answer:

   <pre>function square(n) { return n * n; } square(4);</pre> -> Option C

4. Quick Check:

   Function + call + semicolons = correct REPL test [OK]

Hint: End lines with semicolons and call function last [OK]

Common Mistakes:

• Omitting semicolons causing REPL errors
• Using console.log which prints undefined in REPL
• Not calling the function as last expression