Asynchronous JavaScript Overview

Asynchronous execution in JavaScript refers to code that doesn't run in the order it appears. Instead, it allows tasks to complete independently, while the program moves on to the next task.

The Call Stack keeps track of what function is currently being executed. It follows the principle of Last In First Out (LIFO).

The Callback Queue is a waiting area for tasks that have finished execution but are waiting for a chance to be added to the Call Stack.

The Web API handles tasks like network requests, timers, and user events. These tasks run outside the main thread and are completed independently of the Call Stack.

The Event Loop constantly checks the Callback Queue and moves tasks to the Call Stack when it is empty. It acts as a bridge between the Web API and the Call Stack.

A Callback Function is a function passed as an argument to another function. It's executed when the first function finishes its task.

Callback Hell occurs when multiple nested callbacks are used, making the code difficult to read and maintain. It resembles a pyramid shape with many levels of indentation.

Promise is an object that represents the eventual success or failure of an asynchronous operation. It offers a more structured way to handle asynchronous tasks compared to simple callbacks.

A promise represents an asynchronous operation that can have one of three states: Pending, Fulfilled, or Rejected.

It starts in a Pending state, indicating an ongoing operation. Upon successful completion, it transitions to a Fulfilled state; otherwise, it reaches a Rejected state.

The then() function executes whenever a promise is resolved or rejected. You can use it to handle both successful and failed outcomes of a promise.

The catch() function is specifically invoked if a promise is rejected or encounters an error during execution. It's crucial for handling errors and preventing your code from crashing.

If you're only interested in handling successful outcomes, you can provide only one argument to the then() function. This argument is a callback executed when the promise is resolved.

The then() method always returns a promise, allowing you to chain them together to create a series of asynchronous operations. The results from one then() can be used as input for the next then() method.

The catch() function allows you to provide a callback that handles errors or rejections. It's called after any then() method encounters a failure or error.

In Promises, then() and catch() are crucial functions allowing you to manage successful and unsuccessful outcomes of asynchronous operations. They ensure your code executes reliably and handles both expected and unexpected scenarios.