Try–catch block in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of try–catch with thrown error

What will be printed to the console when this code runs?

Javascript

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

A Caught: Oops!

B Start\nEnd\nCaught: Oops!

C Start\nCaught: Oops!

D Start\nEnd

💡 Hint

Remember that code after throw inside try does not run.

The code prints 'Start' first. Then the error is thrown, so 'End' is skipped. The catch block runs and prints 'Caught: Oops!'.

❓ Predict Output

intermediate

Value of variable after try–catch

What is the value of variable x after this code runs?

Javascript

let x = 0;
try {
  x = 5;
  throw 'error';
  x = 10;
} catch (e) {
  x = 20;
}

A 20

B 5

C 10

D 0

💡 Hint

Check which assignments run before and after the throw.

x is set to 5, then error is thrown, so x=10 is skipped. Catch block sets x to 20.

❓ Predict Output

advanced

Output with nested try–catch and finally

What will this code print to the console?

Javascript

try {
  console.log('Outer try');
  try {
    console.log('Inner try');
    throw 'error';
  } catch (e) {
    console.log('Inner catch');
    throw 'new error';
  } finally {
    console.log('Inner finally');
  }
} catch (e) {
  console.log('Outer catch');
} finally {
  console.log('Outer finally');
}

A Outer try\nInner try\nInner catch\nOuter catch\nOuter finally

B Outer try\nInner try\nInner catch\nOuter catch\nInner finally\nOuter finally

C Outer try\nInner try\nInner finally\nInner catch\nOuter catch\nOuter finally

D Outer try\nInner try\nInner catch\nInner finally\nOuter catch\nOuter finally

💡 Hint

Remember finally always runs after try or catch blocks.

The inner try throws 'error', caught by inner catch which throws 'new error'. Inner finally runs next. Outer catch catches 'new error'. Outer finally runs last.

❓ Predict Output

advanced

Error type caught in catch block

What will be the output of this code?

Javascript

try {
  JSON.parse('invalid json');
} catch (e) {
  console.log(e instanceof SyntaxError);
  console.log(e instanceof Error);
  console.log(e.name);
}

A false\ntrue\nSyntaxError

B true\ntrue\nSyntaxError

C true\nfalse\nError

D false\nfalse\nError

💡 Hint

Check the type of error JSON.parse throws on invalid input.

JSON.parse throws a SyntaxError, which is also an Error. So instanceof SyntaxError and instanceof Error are both true. The name is 'SyntaxError'.

🧠 Conceptual

expert

Behavior of try–catch with asynchronous code

Consider this code snippet. Which statement is true about the output?

Javascript

try {
  setTimeout(() => {
    throw new Error('Async error');
  }, 0);
} catch (e) {
  console.log('Caught:', e.message);
}
console.log('End');

A Prints only 'End' and the error is uncaught

B Prints 'Caught: Async error' then 'End'

C Throws SyntaxError before printing anything

D Prints 'End' then 'Caught: Async error'

💡 Hint

Remember how try–catch works with asynchronous callbacks.

The throw inside setTimeout runs asynchronously, outside the try block's scope. So catch does not catch it. 'End' prints first. The error is uncaught and will appear as an unhandled exception.