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

# Using `finally()` in promise chains
The `finally()` method behaves differently than both `then()` and `catch()` in that it copies the state and value of the previous promise into its returned promise. That means if the original promise is fulfilled with a value, then `finally()` returns a promise that is fulfilled with the same value. For example, we have the following:

const promise = Promise.resolve(11);

promise.finally(() => {
    console.log("Finally called.");
}).then(value => {
    console.log(value);         // 11
});

Here, the settlement handler can’t receive the fulfilled value from promise, so that value is copied to a new promise that is returned from the method call. The new promise is fulfilled with the value `11` (copied from `promise`), so the fulfillment handler receives `11` as an argument. Keep in mind that even though the returned promise and `promise` have the same value, they are not the same object, as we can see in this example:

Discover how the finally() method is used in promise chains.

Introduction

Basics of Promises

Chaining Promises

Working with Multiple Promises

Async Functions and Await Expressions

Tracking Down the Unhandled Rejections

Conclusion

Promise Chains with the finally() Method

Using `finally()` in promise chains

Promise Chains with the finally() Method

Using finally() in promise chains

Using `finally()` in promise chains