JavaScript promises helps us to handle asynchronous operations more efficiently by assigning the handler with the success value or failure reason.

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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

## Assigning handlers with `then()`
The `then()` method is present on all promises and takes two arguments. The first argument is a function to call when the promise is fulfilled, called the **fulfillment handler**. Any additional data related to the asynchronous operation is passed to this function. 

The second argument is a function to call when the promise is rejected, called the **rejection handler**. Similar to the fulfillment handler, the rejection handler is passed any additional data related to the rejection.

> **Note:** Any object that implements the `then()` method in this way is called a **thenable**. All promises are thenables, but not all thenables are promises. 


Let's understand this with the following illustration.

# Assigning handlers with `then()`
The `then()` method is present on all promises and takes two arguments. The first argument is a function to call when the promise is fulfilled, called the **fulfillment handler**. Any additional data related to the asynchronous operation is passed to this function. 

The second argument is a function to call when the promise is rejected, called the **rejection handler**. Similar to the fulfillment handler, the rejection handler is passed any additional data related to the rejection.

> **Note:** Any object that implements the `then()` method in this way is called a **thenable**. All promises are thenables, but not all thenables are promises. 


Let's understand this with the following illustration.

Introduction

Basics of Promises

Chaining Promises

Working with Multiple Promises

Async Functions and Await Expressions

Tracking Down the Unhandled Rejections

Conclusion

Handlers

Assigning handlers with `then()`