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

## Asynchronous functions
Async functions allow us to use promises without manually assigning fulfillment and rejection handlers. We can turn any function into an async function by adding the `async` keyword before the function definition.

The return value of an async function is always a promise. If we return a promise from the async function, then it is duplicated and returned to the call site; if we return a non-promise value, then the value is resolved to a promise and returned to the call site.

## Error handling
Errors thrown inside an async function are caught and returned as rejected promises. Due to this behavior, we can't catch errors originating in an async function using `try-catch`. Instead, we need to assign a rejection handler to the returned promise.

## Iterating with the `for-await-of` loop
Async functions enable two special types of syntax: the `await` expression and the `for-await-of` loop. The `await` expression is used to automatically assign fulfillment and rejection handlers to a promise so that the fulfillment value becomes the return value of the `await` expression and a rejection causes an error to be thrown. Similarly, the `for-await-of` loop operates on an async iterable and allows the use of promises instead of a loop. The `for-await-of` loop waits for each promise returned from the async iterable to fulfill before going on to the next promise. If a promise from the async iterable is rejected, then an error is thrown.

We can use top-level `await` expressions outside of async functions at the top level of JavaScript modules. This capability is not available in scripts.

# Asynchronous functions
Async functions allow us to use promises without manually assigning fulfillment and rejection handlers. We can turn any function into an async function by adding the `async` keyword before the function definition.

The return value of an async function is always a promise. If we return a promise from the async function, then it is duplicated and returned to the call site; if we return a non-promise value, then the value is resolved to a promise and returned to the call site.

# Error handling
Errors thrown inside an async function are caught and returned as rejected promises. Due to this behavior, we can't catch errors originating in an async function using `try-catch`. Instead, we need to assign a rejection handler to the returned promise.

# Iterating with the `for-await-of` loop
Async functions enable two special types of syntax: the `await` expression and the `for-await-of` loop. The `await` expression is used to automatically assign fulfillment and rejection handlers to a promise so that the fulfillment value becomes the return value of the `await` expression and a rejection causes an error to be thrown. Similarly, the `for-await-of` loop operates on an async iterable and allows the use of promises instead of a loop. The `for-await-of` loop waits for each promise returned from the async iterable to fulfill before going on to the next promise. If a promise from the async iterable is rejected, then an error is thrown.

We can use top-level `await` expressions outside of async functions at the top level of JavaScript modules. This capability is not available in scripts.

Get a brief summary of async functions and await expressions.

Introduction

Basics of Promises

Chaining Promises

Working with Multiple Promises

Async Functions and Await Expressions

Tracking Down the Unhandled Rejections

Conclusion

Summary: Async Functions and Await Expressions

Asynchronous functions