Node.js built-in debugger in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

How to start the Node.js built-in debugger?

Which command correctly starts the Node.js built-in debugger for a script named app.js?

A node --inspect app.js

B node --debug-brk app.js

C node debug app.js

D node --inspect-brk app.js

💡 Hint

Think about the original command to launch the built-in CLI debugger.

The node debug app.js command starts the built-in CLI debugger in Node.js. The other options relate to the newer inspector protocol or break on start.

❓ component_behavior

intermediate

What happens when you use debugger; in Node.js code?

If you run a Node.js script with the built-in debugger and the code contains a debugger; statement, what will happen when execution reaches that line?

Node.js

console.log('Start');
debugger;
console.log('End');

A The <code>debugger;</code> line is ignored and execution continues immediately.

B Execution pauses and waits for debugger commands before continuing.

C The script throws a syntax error and stops.

D Execution skips the <code>debugger;</code> line and logs only 'End'.

💡 Hint

Think of debugger; as a breakpoint in the code.

The debugger; statement causes Node.js to pause execution when running under the debugger, allowing inspection and commands before continuing.

📝 Syntax

advanced

Identify the correct command to start Node.js debugger and break on first line

Which command starts the Node.js built-in debugger and pauses execution on the first line of server.js?

A node debug --brk server.js

B node debug --break server.js

C node debug server.js --break

D node --inspect server.js

💡 Hint

Look for the correct flag to break on the first line with the built-in debugger.

The --brk flag tells the debugger to break on the first line. It must come before the script name.

🔧 Debug

advanced

What error occurs if you run node debug without a script?

What error message or behavior occurs when you run node debug without specifying a script file?

A Error: No script specified. Usage: node debug <script.js>

B SyntaxError: Unexpected end of input

C The debugger starts but immediately exits with no message.

D ReferenceError: script is not defined

💡 Hint

Think about what the debugger expects as input.

The built-in debugger requires a script file to debug. Running without one shows a usage error.

❓ state_output

expert

What is the output of this Node.js debugger session?

Given the code below and running node debug test.js, what is the output after typing repl and then x + y in the debugger?

Node.js

const x = 5;
const y = 10;
debugger;
console.log(x + y);

A ReferenceError: x is not defined

B SyntaxError: Unexpected identifier

C undefined

D 15

💡 Hint

Variables declared before the debugger statement are accessible in the REPL.

When paused at debugger;, the REPL can access variables in scope. x + y evaluates to 15.

Practice

1. What is the primary purpose of the Node.js built-in debugger?

easy

A. To pause and inspect code execution to find bugs

B. To speed up the execution of Node.js programs

C. To automatically fix syntax errors in code

D. To deploy Node.js applications to the cloud

Solution

1. Step 1: Understand the debugger's role

   The Node.js built-in debugger allows developers to pause code execution and inspect variables and flow.

2. Step 2: Compare options to debugger purpose

   Only To pause and inspect code execution to find bugs describes pausing and inspecting code to find bugs, which matches the debugger's function.

3. Final Answer:

   To pause and inspect code execution to find bugs -> Option A

4. Quick Check:

   Debugger purpose = pause and inspect [OK]

Hint: Debugger = pause and inspect code to find bugs [OK]

Common Mistakes:

• Thinking debugger speeds up code
• Confusing debugger with auto-fix tools
• Assuming debugger deploys apps

2. Which command correctly starts the Node.js built-in debugger for a script named app.js?

easy

A. node debug app.js

B. node run app.js

C. node inspect app.js

D. node start app.js

Solution

1. Step 1: Recall debugger start commands

   The modern way to start the Node.js debugger is using node inspect followed by the script name.

2. Step 2: Check each option

   node inspect app.js uses node inspect app.js, which is correct. node debug app.js uses deprecated node debug. Options B and D are invalid commands.

3. Final Answer:

   node inspect app.js -> Option C

4. Quick Check:

   Start debugger = node inspect [OK]

Hint: Use 'node inspect' to start debugger, not 'node debug' [OK]

Common Mistakes:

• Using 'node debug' which is deprecated
• Using 'node run' or 'node start' which don't start debugger
• Confusing command syntax order

3. Given this code snippet with debugger statements:

function add(a, b) {
  debugger;
  return a + b;
}

console.log(add(2, 3));

What happens when you run node inspect script.js on this file?

medium

A. The program runs without pausing and prints 5 immediately

B. The program crashes with an exception

C. SyntaxError occurs due to debugger statement

D. Execution pauses at the debugger line before returning the sum

Solution

1. Step 1: Understand debugger statement effect

   The debugger; statement causes the debugger to pause execution at that line when running under a debugger.

2. Step 2: Analyze program flow

   Running node inspect pauses at the debugger; line inside add before returning the sum, allowing inspection.

3. Final Answer:

   Execution pauses at the debugger line before returning the sum -> Option D

4. Quick Check:

   Debugger statement pauses execution under debugger [OK]

Hint: Debugger statement pauses code only when debugger runs [OK]

Common Mistakes:

• Thinking debugger statement causes syntax error
• Assuming program runs without pause under debugger
• Confusing debugger statement with console.log

4. You run node inspect app.js but the debugger does not pause at your debugger; statement inside a function. What is a likely cause?

medium

A. The debugger command is misspelled

B. You forgot to save the file after adding the debugger statement

C. Node.js version does not support debugger

D. Debugger statements only work in browser JavaScript

Solution

1. Step 1: Check file changes

   If the file was not saved after adding debugger;, the running code won't include it, so debugger won't pause.

2. Step 2: Evaluate other options

   Debugger command spelling doesn't affect pause at debugger;. Node.js supports debugger in recent versions. Debugger statements work in Node.js, not only browsers.

3. Final Answer:

   You forgot to save the file after adding the debugger statement -> Option B

4. Quick Check:

   Unsaved file = debugger ignored [OK]

Hint: Always save files before debugging to include changes [OK]

Common Mistakes:

• Assuming debugger statements only work in browsers
• Ignoring file save before running debugger
• Blaming Node.js version without checking

5. You want to debug a Node.js script and step through each line inside a loop. Which command should you use after starting node inspect script.js to move to the next line without entering functions?

hard

A. next

B. cont

C. step

D. out

Solution

1. Step 1: Understand debugger commands

   cont continues running until next breakpoint, step enters functions, next moves to next line without entering functions, out steps out of current function.

2. Step 2: Choose command for stepping through loop lines

   To move line by line in the current function (like inside a loop) without entering called functions, next is the correct command.

3. Final Answer:

   next -> Option A

4. Quick Check:

   Step over lines = next [OK]

Hint: Use 'next' to step over lines without entering functions [OK]

Common Mistakes:

• Using 'cont' which runs without pausing
• Using 'step' which enters functions
• Using 'out' which exits current function