Node.js Promises

What Is a Promise?

A Promise in JavaScript is an object that represents the eventual result of an asynchronous operation. It can be in one of three states:

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

Why Use Promises in Node.js?

• Avoid deeply nested callbacks (callback hell)
• Better code readability and maintainability
• Support for chaining multiple operations
• Easier error handling with .catch()
• Promises are the foundation of async/await

Syntax of a Promise

Example: Basic Promise

Output:

Converting Callback to Promise (Using fs.promises)

Node.js provides promise-based versions of many core APIs like fs.

Promise Chaining

You can chain promises to execute async operations in sequence:

Output:

Handling Errors with .catch()

Errors in a promise chain are caught with .catch():

Promise Static Methods

Example: Promise.all()

Promise.all() runs multiple promises in parallel and waits for all to complete.

Output:

Key Takeaways

• Promises provide a modern solution for asynchronous programming in Node.js
• Improve readability and maintainability over callbacks
• Support chaining via .then() and centralized error handling via .catch()
• Use built-in promise-based Node.js modules like fs.promises
• Combine multiple promises with Promise.all(), Promise.race(), etc.