Catching runtime errors in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of try...catch in JavaScript?

It is used to handle runtime errors gracefully by trying a block of code and catching errors if they occur, preventing the program from crashing.

beginner

What does the catch block receive as its parameter?

The catch block receives the error object that was thrown in the try block, which contains information about the error.

intermediate

How can you throw a custom error in JavaScript?

You can use the throw statement followed by an error object or message, for example: throw new Error('Custom message').

beginner

What happens if an error occurs outside a try...catch block?

The error will be unhandled and usually causes the program to stop or crash, showing an error message in the console.

intermediate

Can you use try...catch to handle asynchronous errors directly?

No, try...catch works for synchronous code. For asynchronous code, you handle errors with catch on promises or use async/await with try...catch.

What keyword starts a block of code where you expect an error might happen?

A try

B catch

C throw

D error

The try block contains code that might throw an error.

Which block runs only if an error occurs in the try block?

A finally

B catch

C throw

D error

The catch block runs when an error is thrown inside the try block.

How do you create a new error to throw?

A try Error('message')

B throw Error('message')

C catch Error('message')

D new Error('message')

You create an error object with new Error('message') and then throw it.

What happens if you don't catch an error?

A The program crashes or stops

B The program continues silently

C The error is ignored

D The error is fixed automatically

Uncaught errors usually cause the program to stop or crash.

Which is the correct way to catch errors from a promise?

A throw promise

B try { promise } catch(error) {}

C promise.catch(error => ...)

D catch(promise)

Promises use the catch method to handle errors asynchronously.

Explain how try...catch helps in handling runtime errors in JavaScript.

Think about what happens when code inside try fails.

Describe how you would handle errors in asynchronous JavaScript code.

Remember that async code needs special handling.

Practice

1.

What is the main purpose of using try and catch in JavaScript?

easy

A. To create new variables

B. To speed up the program execution

C. To handle errors and prevent the program from crashing

D. To repeat code multiple times

Solution

1. Step 1: Understand the role of try block

   The try block runs code that might cause an error.

2. Step 2: Understand the role of catch block

   The catch block handles any error from the try block to keep the program running smoothly.

3. Final Answer:

   To handle errors and prevent the program from crashing -> Option C

4. Quick Check:

   try-catch = error handling [OK]

Hint: Try-catch blocks catch errors to keep programs running [OK]

Common Mistakes:

• Thinking try-catch speeds up code
• Using try-catch to create variables
• Confusing try-catch with loops

2.

Which of the following is the correct syntax to catch errors in JavaScript?

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

easy

A. try { } catch (error) { }

B. try { } catch error() { }

C. try (error) { } catch { }

D. try { } catch error { }

Solution

1. Step 1: Check the try block syntax

   The try block must be followed by catch with parentheses around the error variable.

2. Step 2: Verify the catch block syntax

   The correct syntax is catch (error) { } with parentheses around the error parameter.

3. Final Answer:

   try { } catch (error) { } -> Option A

4. Quick Check:

   catch needs parentheses [OK]

Hint: Catch block always uses parentheses for error [OK]

Common Mistakes:

• Omitting parentheses in catch
• Using parentheses after try
• Adding parentheses after catch keyword

3.

What will be the output of the following code?

try {
  let result = 10 / 0;
  console.log(result);
} catch (e) {
  console.log('Error caught');
}

medium

A. Infinity

B. Error caught

C. 0

D. undefined

Solution

1. Step 1: Understand division by zero in JavaScript

   Dividing 10 by 0 does not cause an error; it results in Infinity.

2. Step 2: Check if catch block runs

   Since no error occurs, the catch block is skipped and Infinity is logged.

3. Final Answer:

   Infinity -> Option A

4. Quick Check:

   10/0 = Infinity, no error [OK]

Hint: Division by zero returns Infinity, no error thrown [OK]

Common Mistakes:

• Assuming division by zero throws error
• Expecting catch block to run
• Confusing 0 with Infinity

4.

Find the error in this code and choose the correct fix:

try {
  console.log(x);
} catch e {
  console.log('Error:', e.message);
}

medium

A. Declare x before try block

B. Change catch e to catch (e)

C. Add finally block after catch

D. Remove the catch block

Solution

1. Step 1: Identify syntax error in catch block

   The catch block must have parentheses around the error variable, so catch e is invalid syntax.

2. Step 2: Correct the catch syntax

   Changing it to catch (e) fixes the syntax and allows error handling.

3. Final Answer:

   Change catch e to catch (e) -> Option B

4. Quick Check:

   Catch needs parentheses around error [OK]

Hint: Catch error variable must be in parentheses [OK]

Common Mistakes:

• Omitting parentheses in catch
• Adding finally when not needed
• Ignoring undeclared variable error

5.

You want to safely parse JSON data that might be invalid. Which code snippet correctly catches parsing errors?

const jsonString = '{"name": "Alice", age: 25}';
try {
  const data = JSON.parse(jsonString);
  console.log(data.name);
} catch (error) {
  console.log('Invalid JSON:', error.message);
}

hard

A. You must use if-else instead of try-catch for JSON parsing

B. This code will crash because JSON.parse never throws errors

C. The catch block should be after console.log(data.name)

D. This code correctly catches JSON parsing errors and logs a message

Solution

1. Step 1: Understand JSON.parse behavior

   JSON.parse throws an error if the JSON string is invalid, like missing quotes around keys.

2. Step 2: Check try-catch usage

   The try block runs JSON.parse, and the catch block handles any parsing error to avoid crashing.

3. Final Answer:

   This code correctly catches JSON parsing errors and logs a message -> Option D

4. Quick Check:

   try-catch handles JSON.parse errors [OK]

Hint: Use try-catch to handle JSON.parse errors safely [OK]

Common Mistakes:

• Assuming JSON.parse never throws errors
• Placing catch block incorrectly
• Using if-else instead of try-catch for errors