Catching runtime errors in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of try-catch with thrown error

What will be the output of this JavaScript code?

Javascript

try {
  throw new Error('Oops!');
} catch (e) {
  console.log('Caught:', e.message);
}
console.log('Done');

A Caught: undefined\nDone

B Done\nCaught: Oops!

C Error: Oops!\nDone

D Caught: Oops!\nDone

💡 Hint

Remember that the catch block runs immediately after the error is thrown.

The throw statement creates an error that is caught by the catch block, which logs 'Caught: Oops!'. Then the program continues and logs 'Done'.

❓ Predict Output

intermediate

Output when error is not caught

What happens when this code runs?

Javascript

function test() {
  JSON.parse('invalid json');
  console.log('Parsed');
}
test();

A undefined

B Parsed

C SyntaxError is thrown and program stops before 'Parsed' logs

D null

💡 Hint

Parsing invalid JSON throws an error if not caught.

JSON.parse throws a SyntaxError on invalid input. Since there is no try-catch, the error stops execution before 'Parsed' logs.

❓ Predict Output

advanced

Output with nested try-catch blocks

What will this code output?

Javascript

try {
  try {
    throw new TypeError('Type issue');
  } catch (e) {
    console.log('Inner catch:', e.name);
    throw e;
  }
} catch (e) {
  console.log('Outer catch:', e.message);
}

A Inner catch: TypeError\nOuter catch: Type issue

B Outer catch: Type issue\nInner catch: TypeError

C Inner catch: TypeError

D TypeError: Type issue

💡 Hint

The inner catch rethrows the error, so the outer catch also runs.

The inner catch logs the error name, then rethrows it. The outer catch catches it again and logs the message.

❓ Predict Output

advanced

Output when finally block modifies return

What is the output of this code?

Javascript

function check() {
  try {
    return 'try';
  } catch {
    return 'catch';
  } finally {
    return 'finally';
  }
}
console.log(check());

A finally

B try

C catch

D undefined

💡 Hint

The finally block runs after try or catch and can override return values.

The finally block always runs last and its return overrides any previous return in try or catch.

❓ Predict Output

expert

Output when catching specific error types

What will this code output?

Javascript

try {
  null.f();
} catch (e) {
  if (e instanceof TypeError) {
    console.log('TypeError caught');
  } else {
    console.log('Other error');
  }
}

A Other error

B TypeError caught

C ReferenceError caught

D No output

💡 Hint

Calling a method on null causes a TypeError.

null.f() throws a TypeError because null has no properties. The catch checks the error type and logs accordingly.

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