How Does JavaScript Work?


JavaScript is a programming language that enables dynamic web pages, providing users with a richer and more interactive experience. It’s one of the most widely used languages in the world, and has become indispensable to modern online life. Without it, websites would be just plain text and images, Google Maps and Facebook wouldn’t look the way they do today, and you wouldn’t have access to the wealth of information and entertainment that is available on the Internet.

JavaScript was first developed in 1995, and has since evolved to be a very versatile scripting language that can be used for a wide range of applications. It’s incredibly popular among front-end developers, but it also has an important role in back end web development, as well.

How Does It Work?

Every major browser loads a web page, parses the HTML and creates a Document Object Model (DOM) from the contents. This DOM then provides the web page’s JavaScript code with a live view of everything on the page, including images and CSS. Once the DOM has a complete view of the page, it then loads the inline JavaScript and other in-line scripts to display them.

Most of the time, JavaScript code is embedded into the HTML of a web page or referenced via an.js file, which is a special type of.js file that doesn’t have a.html extension, like.jsp or.jsm. This allows the JavaScript code to run directly in the browser, rather than having to be processed on the server.

The JavaScript engine translates the code to bytecode, which is an intermediate representation that can be converted into machine code on the computer’s CPU. This is done through a compiler and interpreter. The compiler then performs optimizations to the bytecode, which increases the performance of the JavaScript code.

There are a lot of things that happen in the background when JavaScript is being executed. When the code is compiled, it gets analyzed by a profiler, which determines how much memory each of the lines are using and optimizes them accordingly. This is an essential step in ensuring that JavaScript runs at its best speed.

Once the profiler has finished optimizing the bytecode, it is passed to the compiler, which produces machine code that can be run on the machine’s CPU. This machine code is then fed to the interpreter, which translates it to the real-world language that is being used by the machine’s hardware.

Another thing that happens during the processing of JavaScript code is when functions are called. When the function is called, JavaScript will push it to the top of the stack and it will delegate control of the function to a callback queue if there’s a callback function inside the function. This way, the thread that is running the function can focus on the actual JavaScript code in the body of the function.

It should be noted that JavaScript is single-threaded, so each time a function is called, it’s pushed to the top of the stack and is subsequently executed until its results are displayed on the screen. If a function contains a Web API call, it will be spawned on a separate thread managed by the browser, which means it won’t be executed by the JavaScript main execution thread and is performed outside of the sandbox. This is why the JavaScript debugger is important.