Try-catch for synchronous errors in Node.js - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to catch errors thrown inside the try block.

Node.js

try {
  JSON.parse('invalid json');
} catch ([1]) {
  console.log('Error caught');
}

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

Ae

Berr

Cerror

Dexception

Attempts:

3 left

💡 Hint

Common Mistakes

Leaving the catch parentheses empty

Using a reserved word as the error variable

The catch block receives the error object as a parameter. Here, e is a common short name for the error.

2fill in blank

medium

Complete the code to throw a custom error inside the try block.

Node.js

try {
  throw new [1]('Something went wrong');
} catch (e) {
  console.log(e.message);
}

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

AError

BException

CSyntaxError

DTypeError

Attempts:

3 left

💡 Hint

Common Mistakes

Using Exception which is not a built-in JavaScript error class

Using specific error types when a general error is intended

Error is the base class for errors in JavaScript. Throwing new Error() creates a general error.

3fill in blank

hard

Fix the error in the catch block parameter to properly catch the error.

Node.js

try {
  JSON.parse('bad');
} catch [1] {
  console.log('Caught an error');
}

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

Aerror

B[error]

C(error)

D{error}

Attempts:

3 left

💡 Hint

Common Mistakes

Using square or curly brackets instead of parentheses

Omitting parentheses entirely

The catch block must have parentheses around the error parameter, like (error).

4fill in blank

hard

Fill both blanks to catch a TypeError and log its message.

Node.js

try {
  null.[1]();
} catch (e) {
  if (e instanceof [2]) {
    console.log(e.message);
  }
}

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

AtoString

BTypeError

CReferenceError

Dcall

Attempts:

3 left

💡 Hint

Common Mistakes

Using a method that exists on null

Checking for the wrong error type

Calling toString on null causes a TypeError. The catch block checks for that error type.

5fill in blank

hard

Fill all three blanks to create a try-catch that throws and catches a SyntaxError.

Node.js

try {
  throw new [1]('Invalid syntax');
} catch ([2]) {
  if ([2] instanceof [3]) {
    console.log([2].message);
  }
}

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

ASyntaxError

Berr

CError

De

Attempts:

3 left

💡 Hint

Common Mistakes

Using a generic Error instead of SyntaxError

Using inconsistent variable names in catch and inside the block

The code throws a SyntaxError, catches it as e, and checks if e is an instance of SyntaxError.

Practice

1. What is the main purpose of using try-catch in Node.js?

easy

A. To handle errors that happen during code execution without stopping the program

B. To speed up the execution of asynchronous code

C. To declare variables that are only used inside a block

D. To format output text in the console

Solution

1. Step 1: Understand the role of try-catch

   Try-catch is used to catch errors that happen during the running of code so the program can continue safely.

2. Step 2: Eliminate unrelated options

   Options about speeding asynchronous code, declaring variables, or formatting output do not relate to error handling.

3. Final Answer:

   To handle errors that happen during code execution without stopping the program -> Option A

4. Quick Check:

   Error handling = D [OK]

Hint: Try-catch is for catching errors, not for speed or formatting [OK]

Common Mistakes:

• Thinking try-catch speeds up code
• Confusing try-catch with variable declaration
• Using try-catch to format output

2. Which of the following is the correct syntax to catch synchronous errors in Node.js?

easy

A. catch { /* code */ } try (error) { /* handle error */ }

B. try { /* code */ } catch (error) { /* handle error */ }

C. try: { /* code */ } except (error) { /* handle error */ }

D. try { /* code */ } catch error { /* handle error */ }

Solution

1. Step 1: Recall correct try-catch syntax

   The correct syntax uses try { ... } catch (error) { ... } with parentheses around the error variable.

2. Step 2: Identify syntax errors in other options

   Options A, B, and D have wrong keywords or missing parentheses, which cause syntax errors.

3. Final Answer:

   try { /* code */ } catch (error) { /* handle error */ } -> Option B

4. Quick Check:

   Correct try-catch syntax = C [OK]

Hint: Remember catch always needs parentheses around error [OK]

Common Mistakes:

• Omitting parentheses in catch
• Using wrong keywords like except
• Swapping try and catch blocks

3. What will be the output of this code?

try {
  throw new Error('Oops!');
  console.log('This will not run');
} catch (e) {
  console.log('Caught:', e.message);
}

medium

A. Caught: Oops!

B. This will not run

C. Error: Oops!

D. No output

Solution

1. Step 1: Understand throw inside try block

   The throw statement immediately stops normal execution and jumps to catch.

2. Step 2: Check catch block output

   The catch block logs 'Caught:' plus the error message 'Oops!'. The line after throw is skipped.

3. Final Answer:

   Caught: Oops! -> Option A

4. Quick Check:

   throw triggers catch output = B [OK]

Hint: Throw stops try block; catch runs next [OK]

Common Mistakes:

• Expecting code after throw to run
• Confusing error object with message
• Thinking no output appears

4. Identify the error in this code snippet:

try {
  console.log('Start');
  throw 'Error happened';
} catch e {
  console.log('Caught:', e);
}

medium

A. Console.log syntax is wrong

B. Throw cannot use string errors

C. Missing parentheses around catch parameter

D. Try block must not contain throw

Solution

1. Step 1: Check catch syntax

   The catch block must have parentheses around the error variable, like catch (e).

2. Step 2: Verify other parts

   Throwing a string is allowed, console.log syntax is correct, and throw is valid inside try.

3. Final Answer:

   Missing parentheses around catch parameter -> Option C

4. Quick Check:

   Catch needs parentheses = A [OK]

Hint: Catch always needs parentheses around error variable [OK]

Common Mistakes:

• Omitting parentheses in catch
• Thinking throw can't use strings
• Misreading console.log syntax

5. You want to safely parse JSON input that might be invalid. Which code correctly uses try-catch to handle errors and return null if parsing fails?

hard

A. return JSON.parse(input) || null;

B. if (JSON.parse(input)) {
return JSON.parse(input);
} else {
return null;
}

C. try {
JSON.parse(input);
} catch {
return null;
}

D. try {
return JSON.parse(input);
} catch (e) {
return null;
}

Solution

1. Step 1: Understand JSON.parse error behavior

   JSON.parse throws an error if input is invalid, so we must catch it to avoid crashing.

2. Step 2: Check each option's error handling

   try { return JSON.parse(input); } catch (e) { return null; } uses try-catch correctly and returns null on error. if (JSON.parse(input)) { return JSON.parse(input); } else { return null; } does not catch errors, causing crash. try { JSON.parse(input); } catch { return null; } misses error parameter in catch and does not return parsed value. return JSON.parse(input) || null; does not catch errors and will crash on invalid input.

3. Final Answer:

   try { return JSON.parse(input); } catch (e) { return null; } -> Option D

4. Quick Check:

   Use try-catch to catch JSON errors = A [OK]

Hint: Wrap JSON.parse in try-catch to handle invalid input safely [OK]

Common Mistakes:

• Not using try-catch around JSON.parse
• Ignoring catch error parameter
• Assuming JSON.parse returns null on error