JavaScript promises helps us to handle asynchronous operations more efficiently by assigning the handler with the success value or failure reason.

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JS promises

promises

Promises are used to deal with event handling and callbacks in asynchronous programming in JavaScript, such as clicks and key presses. Promises allow developers to specify a function to execute at some later point in response to certain events. Promises may also indicate whether a certain code succeeded or failed in performing its assigned job.

In this course, you will learn how to chain different promises, which makes the code easier to understand and debug. Next, you’ll learn built-in methods to monitor and respond to multiple promises executing in parallel. Then, you will cover async functions and await expressions that are often used with promises. Finally, you’ll learn how to track promises when they are rejected without a rejection handler.

Promises are important to understand and debug the code in asynchronous programming. The concepts of JS promises help developers working with Node.js and Deno for event handling and with asynchronous JavaScript APIs.

JavaScript Promises


## Tracking unhandled rejections in Node.js
Node.js tracks unhandled promise rejections in a similar, but not identical, way that browsers do. There are two events in Node.js, but these are emitted on the `process` object and have different capitalizations than the browser events:
- `unhandledRejection`: This is emitted when a promise is rejected and no rejection handler is called within one turn of the event loop.
- `rejectionHandled`: This is emitted when a promise is rejected and a rejection handler is called after one turn of the event loop.

Also, unlike the browser events, these events do not receive an event object. The `unhandledRejection` event handler is passed both the rejection reason and the promise that was rejected as arguments. The following code shows `unhandledRejection` in action:


# Tracking unhandled rejections in Node.js
Node.js tracks unhandled promise rejections in a similar, but not identical, way that browsers do. There are two events in Node.js, but these are emitted on the `process` object and have different capitalizations than the browser events:
- `unhandledRejection`: This is emitted when a promise is rejected and no rejection handler is called within one turn of the event loop.
- `rejectionHandled`: This is emitted when a promise is rejected and a rejection handler is called after one turn of the event loop.

Also, unlike the browser events, these events do not receive an event object. The `unhandledRejection` event handler is passed both the rejection reason and the promise that was rejected as arguments. The following code shows `unhandledRejection` in action:

Learn how Node.js detects and reports unhandled rejections.

Introduction

Basics of Promises

Chaining Promises

Working with Multiple Promises

Async Functions and Await Expressions

Tracking Down the Unhandled Rejections

Conclusion

Detecting Unhandled Rejections with Node.js

Tracking unhandled rejections in Node.js