Promise chaining in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Promise chain?

Consider the following Node.js code using Promise chaining. What will be logged to the console?

Node.js

Promise.resolve(5)
  .then(x => x * 2)
  .then(x => { console.log(x); return x + 1; })
  .then(x => console.log(x));

A 11

B 5\n6

C 10

D 10\n11

💡 Hint

Remember each then returns a new Promise with the returned value.

The first then doubles 5 to 10. The second then logs 10 and returns 11. The third then logs 11.

📝 Syntax

intermediate

Which option causes a syntax error in Promise chaining?

Identify which code snippet will cause a syntax error when chaining Promises in Node.js.

A Promise.resolve(1).then(x => { return x + 1 }).then(x => console.log(x));

B Promise.resolve(1).then(x => x + 1).then(x => console.log(x))

C Promise.resolve(1).then(x => x + 1).then(x => { console.log(x) });

D Promise.resolve(1).then(x => x + 1).then(x => console.log(x));

💡 Hint

Look carefully at parentheses and braces.

Option B is missing a closing parenthesis at the end, causing a syntax error.

🔧 Debug

advanced

Why does this Promise chain not log the expected value?

Given the code below, why does the console.log output 'undefined' instead of the expected number?

Node.js

Promise.resolve(3)
  .then(x => { x * 3; })
  .then(x => console.log(x));

A The first then does not return a value, so x is undefined in the next then.

B Promise.resolve(3) is rejected, so then is skipped.

C console.log is called before the Promise resolves.

D The second then has a syntax error causing undefined output.

💡 Hint

Check if the first then returns a value explicitly.

Without a return statement, the first then returns undefined, so the next then receives undefined.

❓ state_output

advanced

What is the final value of 'result' after this Promise chain?

Analyze the code and determine the value of the variable 'result' after the Promise chain completes.

Node.js

let result = 0;
Promise.resolve(2)
  .then(x => x + 3)
  .then(x => { result = x * 2; })
  .then(() => console.log('Done'));

A 10

B 0

C NaN

D 5

💡 Hint

Follow the values through each then and how 'result' is assigned.

2 + 3 = 5, then result = 5 * 2 = 10.

🧠 Conceptual

expert

Which option correctly handles errors in a Promise chain?

Choose the code snippet that properly catches errors occurring in any step of the Promise chain.

A

Promise.resolve()
  .then(() => { throw new Error('Fail'); })
  .then(null, err => console.log('Caught:', err.message));

B

Promise.resolve()
  .then(() => { throw new Error('Fail'); })
  .catch(err => console.log('Caught:', err.message));

C

Promise.resolve()
  .then(() => { throw new Error('Fail'); })
  .then(() => console.log('No error'))
  .catch(err => console.log('Caught:', err.message));

D

Promise.resolve()
  .catch(err => console.log('Caught:', err.message))
  .then(() => { throw new Error('Fail'); });

💡 Hint

Errors thrown in any then should be caught by catch at the end.

Option C throws an error in a then, which is caught by the final catch. Option C also catches but does not continue chaining after error. Option C uses second then argument which only catches errors in previous Promise, not in the chain. Option C's catch is before the error throw, so it won't catch it.