Try-catch for synchronous errors in Node.js - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of a try-catch block in Node.js?

A try-catch block is used to handle errors that happen during the execution of synchronous code. It lets the program continue running instead of crashing.

beginner

Which type of errors can be caught by a try-catch block?

Try-catch blocks catch synchronous errors only. They do not catch errors from asynchronous code like promises or callbacks.

beginner

What happens if an error occurs inside the try block?

If an error happens inside the try block, the code stops running there and jumps to the catch block where you can handle the error safely.

beginner

Show a simple example of try-catch in Node.js to handle a synchronous error.

try {
  const result = JSON.parse('invalid json');
} catch (error) {
  console.log('Caught an error:', error.message);
}

intermediate

Why can't try-catch catch errors from asynchronous code like setTimeout or promises?

Because asynchronous code runs later, outside the current try-catch block's scope. Errors happen after the try-catch has finished, so they need special handling like .catch() or async/await with try-catch.

What does a try-catch block catch in Node.js?

A No errors

B Synchronous errors

C Both synchronous and asynchronous errors

D Asynchronous errors

Try-catch blocks only catch synchronous errors that happen immediately in the code inside the try block.

Where does the program jump when an error occurs inside a try block?

A To the catch block

B To the next line after try-catch

C To the finally block

D It crashes immediately

When an error occurs, the program jumps to the catch block to handle the error.

Which of these errors will NOT be caught by try-catch?

A Error thrown by JSON.parse with invalid input

B Error thrown inside the try block

C Error thrown inside a setTimeout callback

D Error thrown by a function called inside try

Errors in asynchronous callbacks like setTimeout happen later and are not caught by the surrounding try-catch.

What is the correct syntax to catch errors in synchronous code?

A catch { /* handle error */ } try { /* code */ }

B try { /* code */ } then { /* handle error */ }

C try { /* code */ } finally { /* handle error */ }

D try { /* code */ } catch (error) { /* handle error */ }

The try-catch syntax requires try first, then catch with an error parameter.

If you want to handle errors from a promise, what should you use instead of try-catch?

A .catch() method or async/await with try-catch

B Another try-catch block

C setTimeout

D console.log

Promises handle errors with .catch() or async/await combined with try-catch for asynchronous error handling.

Explain how try-catch works for synchronous errors in Node.js and why it cannot catch asynchronous errors.

Think about when the error happens in relation to the try-catch block.

Write a simple Node.js code example using try-catch to handle a synchronous error from JSON parsing.

Use JSON.parse with a wrong string inside try.

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