Node.js built-in debugger in Node.js

Introduction

The Node.js built-in debugger helps you find and fix problems in your code by letting you pause and check what is happening step-by-step.

When your Node.js program is not working as expected and you want to see where it breaks.

When you want to check the value of variables at different points in your code.

When you want to understand how your code runs step-by-step.

When you want to test small parts of your code interactively.

When you want to improve your code by finding hidden bugs.

Syntax

Node.js

node inspect your_script.js

Run this command in your terminal to start the debugger for your script.

You can also start debugging by adding --inspect or --inspect-brk flags for advanced debugging with Chrome DevTools.

Examples

Starts the debugger for the app.js file in the terminal.

Node.js

node inspect app.js

Starts the debugger and pauses on the first line, allowing you to connect a browser debugger.

Node.js

node --inspect-brk app.js

In the debugger prompt, cont resumes running the program after a pause.

Node.js

debug> cont
(debugger continues running the program)

In the debugger prompt, next runs the next line and pauses again.

Node.js

debug> next
(debugger runs the next line of code)

Sample Program

This simple program adds two numbers. The debugger; line pauses the program when running with the debugger, letting you inspect variables.

Node.js

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

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

Important Notes

Use the debugger; statement in your code to set breakpoints easily.

In the debugger prompt, commands like cont, next, step, and repl help you control the program.

You can inspect variables by typing their names in the debugger prompt.

Summary

The Node.js built-in debugger helps pause and inspect your code to find bugs.

Start it with node inspect your_script.js or use debugger; in your code.

Use commands like cont and next to control the program flow while debugging.

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