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.