Why promises are used in Javascript - Challenge Your Understanding

Challenge - 5 Problems

🧠 Conceptual

intermediate

Why do we use promises in JavaScript?

Which of the following best explains why promises are used in JavaScript?

A To handle asynchronous operations and avoid callback hell by chaining actions.

B To make JavaScript code run faster by using multiple CPU cores.

C To store data permanently in the browser's memory.

D To convert synchronous code into asynchronous code automatically.

💡 Hint

Think about how JavaScript handles tasks that take time, like loading data from the internet.

Promises help manage asynchronous tasks by allowing code to run after a task finishes, avoiding deeply nested callbacks.

❓ Predict Output

intermediate

Output of promise chaining

What will be the output of the following JavaScript code?

Javascript

Promise.resolve(5)
  .then(x => x * 2)
  .then(x => x + 3)
  .then(console.log);

A 10

B undefined

C 13

D Error

💡 Hint

Follow the value through each then step.

The promise starts with 5, then multiplies by 2 (10), then adds 3 (13), and finally logs 13.

❓ Predict Output

advanced

Promise error handling output

What will this code print to the console?

Javascript

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

A fail

B Error

C undefined

D recovered

💡 Hint

Look at how the catch changes the flow.

The promise rejects with 'fail', but catch handles it and returns 'recovered', which is logged.

🧠 Conceptual

advanced

Why promises improve code readability

Which statement best describes how promises improve JavaScript code readability?

A They automatically parallelize all asynchronous tasks for faster execution.

B They allow asynchronous code to be written in a linear, easy-to-follow style using chaining.

C They replace all functions with classes for better structure.

D They force all asynchronous code to run synchronously.

💡 Hint

Think about how promises let you write steps one after another.

Promises let you chain asynchronous steps in a clear, linear way, avoiding nested callbacks.

❓ Predict Output

expert

Promise.all behavior with mixed promises

What will be the output of this code?

Javascript

Promise.all([
  Promise.resolve(1),
  Promise.reject('error'),
  Promise.resolve(3)
])
.then(console.log)
.catch(console.log);

A error

B [1, 3]

C [1, 'error', 3]

D undefined

💡 Hint

Remember how Promise.all behaves when one promise rejects.

Promise.all rejects immediately if any promise rejects, so the catch logs 'error'.