Node.js built-in debugger in Node.js - Mini Project: Build & Apply

Using the Node.js Built-in Debugger

📖 Scenario: You are writing a simple Node.js program that calculates the sum of numbers in an array. You want to learn how to use the Node.js built-in debugger to inspect your code step-by-step.

🎯 Goal: Build a Node.js script with a function to sum numbers and set it up so you can debug it using the Node.js built-in debugger.

📋 What You'll Learn

Create an array of numbers called numbers with the values 10, 20, 30, 40, and 50.

Create a variable called total and set it to 0.

Write a for loop using for (const num of numbers) to add each number to total.

Add a debugger; statement inside the loop to pause execution for debugging.

💡 Why This Matters

🌍 Real World

Debugging is a key skill for fixing bugs and understanding how your Node.js code runs step-by-step.

💼 Career

Knowing how to use the Node.js built-in debugger helps developers troubleshoot issues efficiently in real projects.

1

Create the numbers array

Create an array called numbers with these exact values: 10, 20, 30, 40, and 50.

Node.js

// Create the numbers array
// Your code here

Hint

Use square brackets [] to create an array and separate values with commas.

2

Initialize the total variable

Create a variable called total and set it to 0.

Node.js

const numbers = [10, 20, 30, 40, 50];
// Initialize total to 0
// Your code here

Hint

Use let to declare a variable that will change.

3

Add a for loop to sum numbers

Write a for loop using for (const num of numbers) to add each number to total.

Node.js

const numbers = [10, 20, 30, 40, 50];
let total = 0;
// Add a for loop to sum numbers
// Your code here

Hint

Use for (const num of numbers) to loop through each number.

4

Add debugger statement inside the loop

Add a debugger; statement inside the for loop to pause execution for debugging.

Node.js

const numbers = [10, 20, 30, 40, 50];
let total = 0;
for (const num of numbers) {
    // Add debugger statement here
    total += num;
}

Hint

Place debugger; inside the loop block before adding to total.

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