Promises

What is a promise?

A promise is an object that represents the eventual result of an async operation, or more plainly, a value that will be available in the future. It can be in one of three states:

1. Pending — operation hasn't completed yet
2. Fulfilled — operation succeeded (resolved with a value)
3. Rejected — operation failed (rejected with an error)

Once a promise is fulfilled or rejected, it's "settled" and cannot change.

Why promises are better than callbacks

Promises solve the problems with callbacks:

• No callback hell — chain operations with .then() instead of nesting
• Better error handling — use .catch() for all errors
• Easier to read — code flows top to bottom
• Easier to debug — clearer error messages

Creating promises

You can create a promise with new Promise():

const promise = new Promise((resolve, reject) => {
  // Do some async work
  setTimeout(() => {
    resolve("Success!");  // Promise succeeds
  }, 1000);
});

The function you pass to Promise receives two functions:

• resolve(value) — call this when the operation succeeds
• reject(error) — call this when the operation fails

Using promises

Use .then() to handle success and .catch() to handle errors:

const promise = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve("Data received");
  }, 1000);
});

promise
  .then(data => {
    console.log(data);  // "Data received" (runs when promise resolves)
  })
  .catch(error => {
    console.error(error);  // Runs if promise rejects
  });

How .then() works

.then() returns a new promise, which lets you chain operations:

promise
  .then(data => {
    console.log("First:", data);
    return "Processed " + data;  // Return value becomes next promise's value
  })
  .then(processed => {
    console.log("Second:", processed);  // "Processed Data received"
  });

Chaining promises

This is where promises shine! You can chain multiple async operations without nesting:

// With promises - clean and readable!
fetchUser(userId)
  .then(user => {
    return fetchPosts(user.id);  // Return a promise
  })
  .then(posts => {
    return fetchComments(posts[0].id);  // Return another promise
  })
  .then(comments => {
    console.log(comments);  // Clean and readable!
  })
  .catch(error => {
    console.error("Something went wrong:", error);  // One catch for all errors!
  });

Compare this to the callback version—much cleaner!

Shorter chaining syntax

You can make it even shorter by returning promises directly:

fetchUser(userId)
  .then(user => fetchPosts(user.id))      // Return promise directly
  .then(posts => fetchComments(posts[0].id))
  .then(comments => console.log(comments))
  .catch(error => console.error(error));

Error handling

Single .catch() for all errors

One of the best features of promises: a single .catch() handles errors from anywhere in the chain:

fetchUser(userId)
  .then(user => fetchPosts(user.id))
  .then(posts => fetchComments(posts[0].id))
  .then(comments => console.log(comments))
  .catch(error => {
    // This catches errors from ANY step in the chain!
    console.error("Error:", error);
  });

If fetchUser, fetchPosts, or fetchComments fails, the error goes to .catch().

Throwing errors

You can throw errors in .then() handlers, and they'll be caught by .catch():

fetchUser(userId)
  .then(user => {
    if (!user) {
      throw new Error("User not found");  // Throws error
    }
    return fetchPosts(user.id);
  })
  .catch(error => {
    console.error(error);  // Catches the thrown error
  });

Promise methods

Promise.all() — wait for all promises

Use Promise.all() when you need to wait for multiple promises to complete:

const promise1 = fetchUser(1);
const promise2 = fetchUser(2);
const promise3 = fetchUser(3);

Promise.all([promise1, promise2, promise3])
  .then(users => {
    // All promises resolved
    console.log(users);  // [user1, user2, user3]
  })
  .catch(error => {
    // If ANY promise rejects, this runs
    console.error("One failed:", error);
  });

Important: If any promise rejects, Promise.all() rejects immediately.

Promise.allSettled() — wait for all, even if some fail

Use Promise.allSettled() when you want to wait for all promises, even if some fail:

Promise.allSettled([promise1, promise2, promise3])
  .then(results => {
    // All promises settled (fulfilled or rejected)
    results.forEach((result, index) => {
      if (result.status === 'fulfilled') {
        console.log(`Promise ${index} succeeded:`, result.value);
      } else {
        console.log(`Promise ${index} failed:`, result.reason);
      }
    });
  });

Promise.race() — wait for first to complete

Use Promise.race() when you want the first promise that completes (fulfilled or rejected):

Promise.race([slowPromise, fastPromise])
  .then(result => {
    // Whichever completes first
    console.log(result);
  });

Converting callbacks to promises

Many older APIs use callbacks, but you can wrap them in promises:

// Old callback API
function delay(callback, ms) {
  setTimeout(callback, ms);
}

// Wrap it in a promise
function delayPromise(ms) {
  return new Promise(resolve => {
    setTimeout(resolve, ms);
  });
}

// Now use it
delayPromise(1000)
  .then(() => {
    console.log("Waited 1 second!");
  });

Common promise patterns

Creating resolved/rejected promises

Sometimes you need to create a promise that's already resolved or rejected:

// Already resolved
const resolved = Promise.resolve("value");

// Already rejected
const rejected = Promise.reject(new Error("Something went wrong"));

This is useful when you need to return a promise but already have the value.

Summary

• Promises represent future values and have three states: pending, fulfilled, rejected
• Use .then() to handle success and .catch() for errors
• Chain promises to avoid callback hell
• One .catch() handles errors from anywhere in the chain
• Promise.all() waits for all promises; Promise.race() waits for the first
• Promises are better than callbacks, but async/await is even better!