How Does JavaScript Work?

If you’re a web developer, javascript is an essential programming language for adding interactivity and creating animations to websites. Learn how it works and how you can use it to create your own applications.

JavaScript is a scripting language that, when combined with HTML and CSS, forms one of the key trio of standard web technologies. It allows web developers to add dynamic elements to their sites, such as interactive maps and videos. It also gives them the ability to update content in real time, for example displaying different images in a carousel based on user input.

Developed in 1995 by Brendan Eich at Netscape Communications, javascript has since become the de facto language for creating interactive applications on the Web. Its main functions include manipulating the Document Object Model (DOM) and handling events, as well as communicating with servers asynchronously.

A key component of the javascript language is event-driven programming, which uses a chain of events to trigger a function and log or respond to an action. These events can be anything from a click on a button to a page loading, and are handled by function listeners that can register with an element and execute when that element is clicked or other occurrences happen.

During the runtime of a javascript application, the browser first downloads the code in the form of bytes into the machine. This is then parsed to remove any unnecessarily long or complex lines of the code, and the result is an Abstract Syntax Tree (AST). The AST is then translated into executable byte code by an interpreter, which also runs some optimizations, such as inlining (where a piece of a function is placed directly at the call site rather than being executed in line with the rest of the function). This reduces processing time.

The javascript engine then executes the byte code, executing the functions in order. During the execution process, the interpreter checks for errors in the code, manages memory allocation, and reclaims memory occupied by objects and variables no longer in use through garbage collection. It also supports multiple threads, making it possible for a function to execute simultaneously with other functions on the same page without affecting the performance of other pages.

Another important aspect of javascript is its object-oriented programming capabilities, including polymorphism and prototype chains. Polymorphism is the ability of an object to take on many different forms, each with its own set of properties and methods. Prototype chains allow objects to inherit these properties and methods from their parent objects, allowing them to act as templates for other objects of the same type. Both features are crucial for creating interactive applications on the Web, and help keep javascript code clean and readable. JavaScript is also supported by a wide range of tools and libraries, including debuggers, refactoring tools, and static program analysis, such as jSLint and ESLint. This makes it easy to develop and debug web applications, and enables code reuse between platforms.