JavaScript is a popular programming language used all over the world primarily because it is the language of the browser - A web browser is application software for accessing the World Wide Web. When a user requests a web page from a particular website, the web browser retrieves the necessary content from a web server and then displays the page on the user's device. That brief explanation gives as an idea of why JavaScript is so important to the internet.
How Javascript works under the hood is equally important to us programmers because any professional it's important to understand how your tools work on a deeper level. For JavaScript developers it's the DOM what is the DOM? - The Document Object Model (DOM) is the data representation of the objects that comprise the structure and content of a document on the web.
This a great read by Codey Lindley for anyone who wants to take a deep dive into the world of the DOM and come away with a better understand of how it works under the hood.
Check it out free right here