Why debugging skills matter in Node.js - See It in Action

Why debugging skills matter

📖 Scenario: You are building a simple Node.js script that processes a list of numbers to find which are even. Debugging skills help you find and fix mistakes in your code so it works correctly.

🎯 Goal: Create a Node.js script that holds a list of numbers, sets a threshold number, filters the list to keep only even numbers greater than the threshold, and finally exports the filtered list.

📋 What You'll Learn

Create an array called numbers with the values [2, 5, 8, 11, 14]

Create a variable called threshold and set it to 6

Use Array.filter() with a callback that keeps numbers greater than threshold and even

Export the filtered array as filteredNumbers

💡 Why This Matters

🌍 Real World

Filtering data based on conditions is common in real-world apps, like showing only available products or valid user inputs.

💼 Career

Debugging and filtering data are essential skills for Node.js developers working on backend services and APIs.

1

DATA SETUP: Create the numbers array

Create an array called numbers with these exact values: [2, 5, 8, 11, 14]

Node.js

// Create the array called numbers with the values [2, 5, 8, 11, 14]
// Your code here

Hint

Use const numbers = [2, 5, 8, 11, 14]; to create the array.

2

CONFIGURATION: Set the threshold value

Create a variable called threshold and set it to the number 6

Node.js

const numbers = [2, 5, 8, 11, 14];
// Create a variable called threshold and set it to 6
// Your code here

Hint

Use const threshold = 6; to set the threshold.

3

CORE LOGIC: Filter numbers greater than threshold and even

Create a variable called filteredNumbers that uses numbers.filter() with a callback function. The callback should keep numbers greater than threshold and that are even (divisible by 2).

Node.js

const numbers = [2, 5, 8, 11, 14];
const threshold = 6;
// Use numbers.filter() to create filteredNumbers with numbers > threshold and even
// Your code here

Hint

Use numbers.filter(number => number > threshold && number % 2 === 0) to filter the array.

4

COMPLETION: Export the filtered numbers array

Add a line to export filteredNumbers using module.exports.

Node.js

const numbers = [2, 5, 8, 11, 14];
const threshold = 6;
const filteredNumbers = numbers.filter(number => number > threshold && number % 2 === 0);
// Export filteredNumbers using module.exports
// Your code here

Hint

Use module.exports = { filteredNumbers }; to export the variable.

Practice

1. Why is debugging important when writing Node.js programs?

easy

A. It makes the program run faster automatically.

B. It helps find and fix errors to make the program work correctly.

C. It adds new features to the program without coding.

D. It removes the need to write tests for the program.

Solution

1. Step 1: Understand the purpose of debugging

   Debugging is used to find and fix errors in code so the program runs as expected.

2. Step 2: Evaluate the options

   Only It helps find and fix errors to make the program work correctly. correctly states debugging helps fix errors. Other options describe unrelated benefits.

3. Final Answer:

   It helps find and fix errors to make the program work correctly. -> Option B

4. Quick Check:

   Debugging = Fix errors [OK]

Hint: Debugging = finding and fixing errors quickly [OK]

Common Mistakes:

• Thinking debugging improves speed automatically
• Confusing debugging with adding features
• Believing debugging replaces testing

2. Which of the following is the correct way to print a variable's value for debugging in Node.js?

easy

A. console.log(variable);

B. log.console(variable);

C. print(variable);

D. console.print(variable);

Solution

1. Step 1: Recall Node.js debugging syntax

   In Node.js, console.log() is the standard method to print values to the console.

2. Step 2: Check each option

   Only console.log(variable); uses the correct syntax. Others are invalid or do not exist in Node.js.

3. Final Answer:

   console.log(variable); -> Option A

4. Quick Check:

   Print value = console.log() [OK]

Hint: Use console.log() to print variables in Node.js [OK]

Common Mistakes:

• Using console.print() which does not exist
• Using print() without console
• Swapping console and log order

3. What will be the output of this Node.js code snippet?

const x = 5;
console.log(x + y);
const y = 3;

medium

A. 8

B. 5undefined

C. ReferenceError: Cannot access 'y' before initialization

D. NaN

Solution

1. Step 1: Understand variable hoisting with const

   Variables declared with const are not hoisted like var. Accessing before declaration causes ReferenceError.

2. Step 2: Analyze the code execution order

   console.log(x + y); runs before y is declared, causing ReferenceError.

3. Final Answer:

   ReferenceError: Cannot access 'y' before initialization -> Option C

4. Quick Check:

   Access const before declaration = ReferenceError [OK]

Hint: const variables cannot be used before declaration [OK]

Common Mistakes:

• Assuming y is undefined and prints NaN
• Thinking variables are hoisted like var
• Expecting output 8 without error

4. You run a Node.js program and get an unexpected error. Which debugging step helps find the exact line causing the error?

medium

A. Add multiple console.log() statements before and after suspect lines.

B. Delete all code and rewrite from scratch.

C. Ignore the error and run the program again.

D. Change variable names randomly.

Solution

1. Step 1: Understand debugging with console logs

   Adding console.log() before and after lines helps track program flow and locate errors.

2. Step 2: Evaluate other options

   Deleting code or ignoring errors does not help find the error. Random renaming causes more issues.

3. Final Answer:

   Add multiple console.log() statements before and after suspect lines. -> Option A

4. Quick Check:

   Use console.log() to trace errors [OK]

Hint: Trace errors with console.log() around problem code [OK]

Common Mistakes:

• Ignoring errors hoping they disappear
• Deleting code without understanding
• Changing variable names without reason

5. You have a Node.js function that sometimes returns undefined unexpectedly. What is the best debugging approach to find why?

hard

A. Comment out the entire function and run the program.

B. Remove all return statements to avoid undefined.

C. Restart the computer to clear errors.

D. Use console.log() to print input and output values at each step inside the function.

Solution

1. Step 1: Trace function inputs and outputs

   Printing inputs and outputs inside the function helps identify where undefined is introduced.

2. Step 2: Avoid ineffective fixes

   Removing returns or commenting out the function hides the problem. Restarting does not fix code logic errors.

3. Final Answer:

   Use console.log() to print input and output values at each step inside the function. -> Option D

4. Quick Check:

   Trace values inside function with console.log() [OK]

Hint: Log inputs and outputs inside functions to find undefined [OK]

Common Mistakes:

• Removing return statements causing more bugs
• Restarting computer instead of debugging code
• Commenting out code without fixing logic