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

# Returning promises in promise chains
Returning primitive values from promise handlers allows the passing of data between promises, but what if we return an object? If the object is a promise, then there’s an extra step that’s taken to determine how to proceed. Consider the following example:

const promise1 = Promise.resolve(11);
const promise2 = Promise.resolve(22);

promise1.then(value => {
    console.log(value);     // 11
    return promise2;
}).then(value => {
    console.log(value);     // 22
});

In this code, `promise1` resolves to `11`. The fulfillment handler for `promise1` returns `promise2`, a promise already in the resolved state. The second fulfillment handler is called because `promise2` has been fulfilled. If `promise2` were rejected, a rejection handler (if present) would be called instead of the second fulfillment handler.

The important thing to recognize about this pattern is that the second fulfillment handler is not added to `promise2` but rather to a third promise, making the previous example equivalent to this:

Learn how to return a promise from a settlement handler using then() and catch().

Introduction

Basics of Promises

Chaining Promises

Working with Multiple Promises

Async Functions and Await Expressions

Tracking Down the Unhandled Rejections

Conclusion

Return Promises in Promise Chains

Returning promises in promise chains