Promises and Async/Await

Promises and async/await provide modern, cleaner ways to handle asynchronous operations in JavaScript and Node.js, replacing deeply nested callbacks.

1. What Is a Promise?

A Promise is an object that represents the eventual result of an asynchronous operation.

A Promise can be in one of three states:

• Pending – operation in progress

• Fulfilled – operation completed successfully

• Rejected – operation failed

2. Creating a Promise

const fetchData = new Promise((resolve, reject) => {
  const success = true;

  if (success) {
    resolve('Data received');
  } else {
    reject('Error occurred');
  }
});

3. Consuming a Promise

fetchData
  .then(result => {
    console.log(result);
  })
  .catch(error => {
    console.error(error);
  })
  .finally(() => {
    console.log('Operation completed');
  });

4. Promise Chaining

Promises can be chained to avoid callback nesting.

getUser()
  .then(user => getOrders(user))
  .then(orders => processOrders(orders))
  .catch(err => console.error(err));

5. Promise Utility Methods

6. Async / Await

async/await is syntactic sugar built on top of promises, making asynchronous code look synchronous.

6.1 Using async and await

async function fetchData() {
  try {
    const result = await fetchPromise();
    console.log(result);
  } catch (error) {
    console.error(error);
  }
}

✔ Easier to read
✔ Easier to debug

7. Error Handling with Async/Await

try {
  await riskyTask();
} catch (err) {
  console.log(err.message);
}

8. Parallel Execution with Async/Await

const [a, b] = await Promise.all([task1(), task2()]);

9. Promises vs Async/Await

10. Common Mistakes

❌ Forgetting await
❌ Not handling errors
❌ Using await inside loops incorrectly

11. Best Practices

• Always handle errors

• Use Promise.all() for parallel tasks

• Prefer async/await for readability

• Avoid mixing callbacks with promises

12. Summary

• Promises manage asynchronous results

• Async/await simplifies promise handling

• Improves code clarity and maintainability

• Essential for modern Node.js development