REPL for interactive exploration in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Node.js REPL code snippet?

Consider this code entered in the Node.js REPL. What will be the output after pressing Enter?

Node.js

const a = 5;
const b = 10;
a + b;

A 15

B undefined

C NaN

D SyntaxError

💡 Hint

Remember that the REPL evaluates expressions and shows their result.

In the Node.js REPL, expressions are evaluated and their result is printed. Here, a + b equals 15.

❓ component_behavior

intermediate

What happens when you declare a variable without var, let, or const in Node.js REPL?

In the Node.js REPL, what is the behavior of this code snippet? x = 42; x;

Node.js

x = 42;
x;

A SyntaxError

B ReferenceError: x is not defined

C undefined

D 42

💡 Hint

In REPL, assigning without declaration creates a global variable.

In the Node.js REPL, assigning a value to an undeclared variable creates a global variable. So x becomes 42.

📝 Syntax

advanced

Which option causes a SyntaxError in Node.js REPL?

Which of these code snippets will cause a SyntaxError when entered in the Node.js REPL?

A const x = ;

B function test() { return 1; }

C let y = 5;

D console.log('Hello');

💡 Hint

Look for incomplete or invalid syntax.

Option A has an incomplete variable declaration 'const x = ;' which is invalid syntax and causes a SyntaxError.

🔧 Debug

advanced

Why does this REPL code throw a ReferenceError?

In the Node.js REPL, why does this code throw a ReferenceError? console.log(myVar); let myVar = 10;

Node.js

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

A Because the REPL does not support let declarations

B Because console.log is not defined in REPL

C Because myVar is used before declaration and let does not hoist initialization

D Because myVar is declared with let and cannot be logged

💡 Hint

Think about variable hoisting and temporal dead zone.

Variables declared with let are hoisted but not initialized, so accessing them before declaration causes ReferenceError.

🧠 Conceptual

expert

What is the purpose of the underscore (_) variable in Node.js REPL?

In the Node.js REPL, what does the underscore (_) variable represent?

A It is used to clear the REPL screen

B It stores the result of the last evaluated expression

C It is a special variable for declaring constants

D It holds the current working directory path

💡 Hint

Try evaluating expressions and then typing _ to see what it holds.

The underscore variable in Node.js REPL automatically stores the value of the last expression evaluated.

Practice

1. What does the Node.js REPL allow you to do?

easy

A. Create graphical user interfaces

B. Compile JavaScript files into executables

C. Run JavaScript code interactively line by line

D. Deploy Node.js applications to servers

Solution

1. Step 1: Understand REPL purpose

   REPL stands for Read-Eval-Print Loop, which lets you type and run code interactively.

2. Step 2: Identify correct REPL use

   It is used to run JavaScript commands one by one and see results immediately.

3. Final Answer:

   Run JavaScript code interactively line by line -> Option C

4. Quick Check:

   REPL = Interactive code execution [OK]

Hint: REPL runs code interactively, not for compiling or deploying [OK]

Common Mistakes:

• Confusing REPL with compiling or deployment tools
• Thinking REPL creates GUIs
• Assuming REPL runs entire scripts automatically

2. Which command starts the Node.js REPL in your terminal?

easy

A. node

B. node repl

C. npm repl

D. node start

Solution

1. Step 1: Recall how to launch REPL

   Typing node alone in the terminal starts the REPL environment.

2. Step 2: Check other options

   Commands like node start, npm repl, or node repl are invalid or do not start REPL.

3. Final Answer:

   node -> Option A

4. Quick Check:

   Start REPL with 'node' command [OK]

Hint: Just type 'node' to enter REPL, no extra words needed [OK]

Common Mistakes:

• Adding extra words after 'node' command
• Using npm commands to start REPL
• Typing 'node repl' which is invalid

3. What will be the output in Node.js REPL after entering 2 + 3 * 4?

medium

A. 20

B. 14

C. 24

D. Error

Solution

1. Step 1: Apply operator precedence

   Multiplication (*) has higher precedence than addition (+), so 3 * 4 = 12 first.

2. Step 2: Calculate final expression

   Then add 2 + 12 = 14.

3. Final Answer:

   14 -> Option B

4. Quick Check:

   2 + (3 * 4) = 14 [OK]

Hint: Multiply before adding to get correct result [OK]

Common Mistakes:

• Adding 2 + 3 first, then multiplying
• Expecting an error for simple math
• Confusing multiplication and addition order

4. You typed const x 5 in Node.js REPL but got a syntax error. What is the likely cause?

medium

A. Missing semicolon at the end

B. Using const inside REPL is not allowed

C. You forgot to press Enter after typing

D. You missed the equal sign (=) in the declaration

Solution

1. Step 1: Check the code syntax

   The code const x 5 is missing the equal sign (=) between variable name and value.

2. Step 2: Understand REPL syntax rules

   REPL accepts const declarations with proper syntax; semicolon is optional, and pressing Enter is required.

3. Final Answer:

   You missed the equal sign (=) in the declaration -> Option D

4. Quick Check:

   const needs '=' to assign value [OK]

Hint: Check for '=' in variable declarations [OK]

Common Mistakes:

• Assuming semicolon is mandatory
• Thinking const is disallowed in REPL
• Not pressing Enter to execute

5. In Node.js REPL, how can you quickly exit the interactive session?

hard

A. Type exit or press Ctrl + C twice

B. Type quit() and press Enter

C. Close the terminal window only

D. Press Ctrl + Z once

Solution

1. Step 1: Recall REPL exit commands

   You can exit REPL by typing exit or pressing Ctrl + C twice quickly.

2. Step 2: Evaluate other options

   quit() is not recognized, closing terminal is abrupt, and Ctrl + Z suspends process but does not exit REPL.

3. Final Answer:

   Type exit or press Ctrl + C twice -> Option A

4. Quick Check:

   Exit REPL with 'exit' or Ctrl+C twice [OK]

Hint: Use 'exit' or double Ctrl+C to quit REPL fast [OK]

Common Mistakes:

• Using quit() which is invalid
• Pressing Ctrl+Z expecting exit
• Closing terminal instead of proper exit