Node.js built-in debugger in Node.js - Cheat Sheet & Quick Revision

Recall & Review

beginner

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

The Node.js built-in debugger helps you find and fix errors in your code by letting you pause execution, inspect variables, and step through your program line by line.

beginner

How do you start the Node.js built-in debugger from the command line?

You start it by running your script with the command node inspect your_script.js. This launches the debugger interface in the terminal.

intermediate

What command do you use in the Node.js debugger to continue execution until the next breakpoint?

Use the cont or c command to continue running until the next breakpoint or pause point.

intermediate

Explain the use of the repl command in the Node.js debugger.

The repl command opens a Read-Eval-Print Loop where you can type JavaScript expressions to inspect or change variables while paused.

beginner

How can you set a breakpoint in your Node.js code to use with the built-in debugger?

Insert the statement debugger; in your code where you want to pause. When running with the debugger, execution will stop at that line.

Which command starts the Node.js built-in debugger?

A node inspect script.js

B node debug script.js

C node run script.js

D node start script.js

The correct command is node inspect script.js to start the built-in debugger.

What does the cont command do in the Node.js debugger?

A Restarts the program

B Stops the debugger

C Prints all variables

D Continues execution until the next breakpoint

The cont command resumes running the program until it hits the next breakpoint or finishes.

How do you pause execution at a specific line in your code when debugging?

A Add <code>pause;</code> in code

B Add <code>debugger;</code> in code

C Add <code>stop;</code> in code

D Add <code>break;</code> in code

The debugger; statement pauses execution when the debugger is running.

What is the purpose of the repl command in the Node.js debugger?

A Runs the program

B Exits the debugger

C Allows interactive JavaScript evaluation

D Sets a breakpoint

The repl command opens an interactive prompt to evaluate JavaScript expressions while paused.

Which of these is NOT a feature of the Node.js built-in debugger?

A Automatically fix bugs

B Inspect variable values

C Step through code line by line

D Set breakpoints

The debugger helps find bugs but does not automatically fix them.

Describe how to use the Node.js built-in debugger to find a bug in your code.

Think about starting, pausing, stepping, and checking values.

Explain the role of the debugger; statement in Node.js debugging.

It's a special line you add to pause your program.

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