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
Multiple promises can be chained together in a variety of ways to pass information between them. Each call to `then()`, `catch()`, and `finally()` creates and returns a new promise that is resolved when the preceding promise is settled. If the promise handler returns a value, then that value becomes the value of the newly created promise from `then()` and `catch()` (`finally()` ignores this value). If the promise handler throws an error, then the error is caught and the returned, newly created promise is rejected using that error as the reason.

# Handling rejections in promise chains
When one promise is rejected in a chain, the promises created from other chained handlers are also rejected until the end of the chain is reached. Knowing this, it’s recommended to attach a rejection handler at the end of each promise chain to ensure that errors are handled correctly. Failing to catch a promise rejection will result in a message being output to the console, an error being thrown, or both (depending on the runtime environment).

# Returning promises from handlers

We can return promises from handlers, and in that case, the promise returned from the call to `then()` and `catch()` will settle to match the settlement state and value of the promise returned from the handler (fulfilled promises returned from `finally()` are ignored, while rejected promises are honored). We can use this to our advantage by delaying some operations until a promise is fulfilled, then initiating and returning a second promise to continue using the same promise chain.

Get a quick summary of chaining promises.

Introduction

Basics of Promises

Chaining Promises

Working with Multiple Promises

Async Functions and Await Expressions

Tracking Down the Unhandled Rejections

Conclusion

Summary: Chaining Promises