Promises for cleaner async in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Promise chain?

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

Node.js

Promise.resolve(5)
  .then(x => x * 2)
  .then(x => { throw new Error('Fail'); })
  .catch(() => 10)
  .then(x => x + 1)
  .then(console.log);

A 11

B Error: Fail

C 6

D undefined

💡 Hint

Remember that catch returns a resolved Promise with the value it returns.

The chain starts with 5, doubles to 10, then throws an error. The catch handles the error and returns 10. Then 1 is added, resulting in 11, which is logged.

❓ component_behavior

intermediate

Which option correctly returns a Promise that resolves after 1 second?

You want to create a function that returns a Promise resolving with 'done' after 1 second. Which code does this correctly?

A function wait() { return Promise.resolve(setTimeout(() => 'done', 1000)); }

B function wait() { setTimeout(() => Promise.resolve('done'), 1000); }

C function wait() { return new Promise(resolve => setTimeout(() => resolve('done'), 1000)); }

D function wait() { return new Promise(resolve => resolve(setTimeout(() => 'done', 1000))); }

💡 Hint

Remember that setTimeout returns a timer ID, not a Promise.

Option C correctly creates a Promise and resolves it after 1 second using setTimeout. Other options either don't return a Promise or resolve immediately.

🔧 Debug

advanced

Why does this Promise chain never log anything?

Look at this code snippet. Why does it never log 'Finished'?

Node.js

Promise.resolve()
  .then(() => { throw 'Error happened'; })
  .then(() => console.log('Finished'));

A Because the Promise never resolves.

B Because console.log is inside a then that never runs due to syntax error.

C Because the Promise chain is missing a return statement.

D Because the error is thrown but not caught, so the last then is skipped.

💡 Hint

Think about what happens when a Promise rejects and no catch is present.

The thrown error causes the Promise to reject. Without a catch, the next then is skipped, so 'Finished' is never logged.

📝 Syntax

advanced

Which option correctly chains Promises to fetch data and process it?

Given fetchData() returns a Promise resolving to data, which code correctly fetches and logs the processed data?

Node.js

function process(data) { return data.length; }

A fetchData().then(data => process(data)).then(result => console.log(result));

B fetchData().then(process).then(console.log);

C fetchData().then(process()).then(console.log);

D fetchData().then(data => { process(data); }).then(console.log);

💡 Hint

Remember to pass functions, not call them immediately in then.

Options A and B correctly pass functions to then. Option C calls process immediately, D does not return from then.

🧠 Conceptual

expert

What is the behavior of Promise.all with mixed resolved and rejected Promises?

Given these Promises: p1 resolves, p2 rejects, p3 resolves. What does Promise.all([p1, p2, p3]) do?

A It resolves with an array of all results including the rejection reason.

B It immediately rejects with p2's rejection reason, ignoring p1 and p3 results.

C It waits for all Promises and then resolves with only the successful results.

D It resolves with the first resolved Promise's value.

💡 Hint

Promise.all rejects as soon as any Promise rejects.

Promise.all rejects immediately when any Promise rejects, with that rejection reason.