Promises

What is a Promise?

A Promise is an object that represents the eventual completion (or failure) of an asynchronous operation. It's a way to handle asynchronous code without callback hell.

Promise States

• Pending: Initial state, not yet fulfilled or rejected
• Fulfilled: Operation completed successfully
• Rejected: Operation failed

Creating a Promise

const promise = new Promise((resolve, reject) => {
  // Asynchronous operation
  if (success) {
    resolve(result); // Fulfilled
  } else {
    reject(error); // Rejected
  }
});

Using Promises

// With .then() and .catch()
promise
  .then(result => console.log(result))
  .catch(error => console.error(error));

// With async/await (covered next lesson)
async function handlePromise() {
  try {
    const result = await promise;
    console.log(result);
  } catch (error) {
    console.error(error);
  }
}

Promise Chaining

Promises can be chained to run sequential async operations.

fetchUser()
  .then(user => fetchProfile(user.id))
  .then(profile => console.log(profile))
  .catch(error => console.error(error));

Promise Methods

• Promise.all() — waits for all promises to resolve
• Promise.race() — resolves with the first promise to settle
• Promise.allSettled() — waits for all promises to settle
• Promise.resolve() — returns a resolved promise
• Promise.reject() — returns a rejected promise

Common Mistakes

• Forgetting to return a promise in a chain (breaks the chain)
• Not handling errors with .catch()
• Creating promises that don't actually resolve or reject
• Using Promise.all() when a single failure should not stop everything

Quick Quiz

Summary

• Promises represent asynchronous operations
• States: Pending, Fulfilled, Rejected
• Use .then() for success, .catch() for errors
• Chain promises for sequential operations
• Promise.all() waits for all promises
• Always handle errors with .catch()