Gain insights into JavaScript Promises to handle asynchronous events, chain multiple promises, utilize built-in methods, and master async functions with await expressions for efficient event handling.



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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

## Promise chains returning promises

Returning a promise from a settlement handler using `finally()` also exhibits some different behavior than using `then()` or `catch()`. First, if we return a fulfilled promise from a settlement handler, then that promise is ignored in favor of the value from the original promise, as in this example:



const promise = Promise.resolve(11);

promise.finally(() => {
    return Promise.resolve(22);
}).then(value => {
    console.log(value);     // 11
});

In this example, the settlement handler returns a promise that is fulfilled with `22`, but
the returned promise is fulfilled with the original promise’s value, which is `11`. 

However, if we return a rejected promise from a settlement handler, then the
returned promise adopts that reason and the returned promise is rejected, like this:

# Promise chains returning promises

Returning a promise from a settlement handler using `finally()` also exhibits some different behavior than using `then()` or `catch()`. First, if we return a fulfilled promise from a settlement handler, then that promise is ignored in favor of the value from the original promise, as in this example:



Learn how to return a promise from a settlement handler using finally().

Introduction

Basics of Promises

Chaining Promises

Working with Multiple Promises

Async Functions and Await Expressions

Tracking Down the Unhandled Rejections

Conclusion

Return Promises from finally()

Promise chains returning promises