Why debugging skills matter in Node.js - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to print an error message when a promise is rejected.

Node.js

fetch('https://api.example.com/data')
  .then(response => response.json())
  .catch(error => console.[1](error))

Drag options to blanks, or click blank then click option'

Alog

Binfo

Cwarn

Derror

Attempts:

3 left

💡 Hint

Common Mistakes

Using console.log instead of console.error

Using console.warn which is less clear for errors

Use console.error to clearly show error messages in Node.js debugging.

2fill in blank

medium

Complete the code to catch and log errors in an async function.

Node.js

async function getData() {
  try {
    const response = await fetch('https://api.example.com/data');
    const data = await response.json();
    return data;
  } catch ([1]) {
    console.error(error);
  }
}

Drag options to blanks, or click blank then click option'

Aerr

Be

Cerror

Dexception

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different variable name than the one logged

Leaving the catch parameter empty

The catch block uses error as the variable name to hold the caught error.

3fill in blank

hard

Fix the error in the code to properly handle rejected promises.

Node.js

fetch('https://api.example.com/data')
  .then(response => response.json())
  .then(data => console.log(data))
  .[1](error => console.error(error))

Drag options to blanks, or click blank then click option'

Aerror

Bcatch

Cfail

Dreject

Attempts:

3 left

💡 Hint

Common Mistakes

Using .error or .fail which are not valid promise methods

Using .reject which is not a method

The correct method to handle rejected promises is catch.

4fill in blank

hard

Fill both blanks to create a simple error handler function.

Node.js

function handleError([1]) {
  console.[2](error.message);
}

Drag options to blanks, or click blank then click option'

Aerror

Blog

Dwarn

Attempts:

3 left

💡 Hint

Common Mistakes

Using console.log instead of console.error

Mismatching parameter and logged variable names

The function takes error as parameter and logs the message using console.error.

5fill in blank

hard

Fill all three blanks to log error details conditionally.

Node.js

function logErrorDetails([1]) {
  if (error.[2]) {
    console.[3](`Error: ${error.message}`);
  }
}

Drag options to blanks, or click blank then click option'

Aerror

Bstack

Dlog

Attempts:

3 left

💡 Hint

Common Mistakes

Using console.log instead of console.error

Checking a wrong property instead of 'stack'

The function checks if the error has a stack trace and logs the message using console.error.

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