Then and catch methods in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Promise chaining with then and catch

What is the output of this code snippet?

Javascript

Promise.resolve(5)
  .then(x => x * 2)
  .then(x => { throw new Error('Oops'); })
  .catch(e => 'Error caught')
  .then(x => x + ' done')
  .then(console.log);

A Uncaught Error

B Oops done

C 10 done

D Error caught done

💡 Hint

Remember catch handles errors and passes its return value to the next then.

The promise resolves with 5, then multiplied by 2 gives 10. Then an error is thrown, which is caught by catch returning 'Error caught'. The next then appends ' done' to that string, resulting in 'Error caught done'.

❓ Predict Output

intermediate

Error propagation in promise chains

What will be logged by this code?

Javascript

Promise.reject('fail')
  .then(() => 'success')
  .catch(err => err)
  .then(console.log);

A success

B Uncaught Error

C fail

D undefined

💡 Hint

Catch returns the error string, which is passed to the next then.

The promise is rejected with 'fail'. The then is skipped. The catch receives 'fail' and returns it. The next then logs 'fail'.

❓ Predict Output

advanced

Multiple catch blocks in promise chain

What is the output of this code?

Javascript

Promise.resolve('start')
  .then(val => { throw 'error1'; })
  .catch(err => {
    console.log('Caught:', err);
    throw 'error2';
  })
  .catch(err => console.log('Caught again:', err));

A Caught: error1

B Caught: error1\nCaught again: error2

C Caught again: error2

D Uncaught error1

💡 Hint

A catch can throw again to be caught by another catch.

The first then throws 'error1', caught by first catch which logs it and throws 'error2'. The second catch catches 'error2' and logs it.

🧠 Conceptual

advanced

Promise then return values

Which option correctly describes what happens when a then callback returns a promise?

A The next then waits for the returned promise to resolve before running.

B The next then runs immediately ignoring the returned promise.

C The returned promise is ignored and the next then receives undefined.

D The chain breaks and no further thens run.

💡 Hint

Think about how promise chaining works with promises returned inside then.

When a then returns a promise, the next then waits for that promise to resolve and receives its resolved value.

❓ Predict Output

expert

Complex promise chain with nested then and catch

What is the final output logged by this code?

Javascript

Promise.resolve(1)
  .then(x => x + 1)
  .then(x => {
    return Promise.reject('fail at step 3');
  })
  .catch(err => {
    console.log('Caught:', err);
    return 10;
  })
  .then(x => x * 2)
  .then(x => { throw 'final error'; })
  .catch(err => console.log('Final catch:', err));

A Caught: fail at step 3\nFinal catch: final error

B Final catch: final error

C Caught: fail at step 3

D Uncaught final error

💡 Hint

Follow the chain carefully, noting where errors are caught and what values are returned.

The promise resolves with 1, then adds 1 to get 2. Then it rejects with 'fail at step 3'. The catch logs 'Caught: fail at step 3' and returns 10. Next then multiplies 10 by 2 to get 20. Then throws 'final error'. The last catch logs 'Final catch: final error'.