JavaScript Promises Interview Questions & Answers

Promises are a core feature of modern JavaScript for handling asynchronous operations. They provide a cleaner, more flexible alternative to callbacks and are foundational for async/await.

Table of Contents

• What is a Promise?
• Promise States
• Creating Promises
• Consuming Promises
• Promise Chaining
• Error Handling
• Promise Utilities
• Common Pitfalls
• Interview Questions

What is a Promise?

A Promise is an object representing the eventual completion (or failure) of an asynchronous operation and its resulting value.

• Promises help avoid “callback hell” and make async code easier to read and maintain.

Promise States

A Promise can be in one of three states:

Creating Promises

const promise = new Promise((resolve, reject) => {
  // async operation
  if (/* success */) {
    resolve('Result');
  } else {
    reject('Error');
  }
});

Example:

function asyncTask(success) {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      if (success) {
        resolve('Task completed!');
      } else {
        reject('Task failed!');
      }
    }, 1000);
  });
}

Consuming Promises

• Use .then() for success, .catch() for errors, and .finally() for cleanup.

asyncTask(true)
  .then(result => console.log(result))
  .catch(error => console.error(error))
  .finally(() => console.log('Done!'));

Promise Chaining

• Promises can be chained for sequential async operations.

asyncTask(true)
  .then(result => {
    console.log(result);
    return asyncTask(false);
  })
  .then(result => console.log(result))
  .catch(error => console.error('Caught:', error));

Error Handling

• Errors thrown in .then() are caught by the next .catch().
• Always end promise chains with .catch() to handle errors.

asyncTask(false)
  .then(result => {
    throw new Error('Something went wrong!');
  })
  .catch(error => console.error(error));

Promise Utilities

• Promise.all([p1, p2, ...]) – Waits for all promises to resolve or any to reject.
• Promise.race([p1, p2, ...]) – Resolves/rejects as soon as one promise settles.
• Promise.allSettled([p1, p2, ...]) – Waits for all to settle (fulfilled or rejected).
• Promise.any([p1, p2, ...]) – Resolves as soon as one fulfills, rejects if all reject.

Example:

const p1 = Promise.resolve(1);
const p2 = Promise.resolve(2);
const p3 = Promise.reject('fail');

Promise.all([p1, p2])
  .then(values => console.log(values)); // [1, 2]

Promise.race([p1, p2])
  .then(value => console.log(value)); // 1

Promise.allSettled([p1, p3])
  .then(results => console.log(results));
// [{status: 'fulfilled', value: 1}, {status: 'rejected', reason: 'fail'}]

Promise.any([p3, p1])
  .then(value => console.log(value)); // 1

Common Pitfalls

• Not returning promises in chains
• Forgetting to handle errors
• Mixing callbacks and promises
• Creating unhandled promise rejections

Interview Questions

1. What is a Promise and why is it useful?
   • A Promise is an object representing the eventual result of an async operation. It helps manage async code and avoid callback hell.
2. What are the three states of a Promise?
   • pending, fulfilled, rejected
3. How do you create a Promise?
   • Using new Promise((resolve, reject) => { ... })
4. How do you consume a Promise?
   • Using .then(), .catch(), and .finally()
5. What is promise chaining?
   • Linking multiple .then() calls for sequential async operations.
6. How do you handle errors in promises?
   • Using .catch() or the second argument of .then()
7. What does Promise.all do?
   • Waits for all promises to resolve or any to reject.
8. What is the difference between Promise.all and Promise.race?
   • all waits for all, race resolves/rejects as soon as one settles.
9. What is an unhandled promise rejection?
   • When a promise is rejected and no .catch() handles the error.
10. How do you convert a callback-based function to a promise-based one?

• Use the Promise constructor or util.promisify in Node.js.

Additional Resources

• MDN Promises
• JavaScript.info: Promises

Mastering Promises is essential for modern JavaScript development and interview success.