How Does javascript Work?

Whether you’re working on front-end or back-end, there’s a good chance that javascript will come into play somewhere in the mix. The popular scripting language empowers developers to add functionality to web pages, allowing them to make a site interactive and responsive. It’s what powers everything from the way that Facebook updates your timeline or how Google suggests search terms as you start typing. It’s hard to imagine the modern web without these features, and javascript is at the heart of them all.

Despite how complicated it might seem, javascript is actually quite simple. It’s based on the same core concepts as other languages, with a similar structure and programming model. But it also offers some important differences, notably its support for dynamic typing and a prototype-based object system (instead of the class-based one you may be more familiar with from other languages). And because of this, javascript runs faster than others, with fewer bugs and errors to deal with.

In order to run javascript, it needs to be interpreted by a web browser. A web browser is a little like a factory, taking in raw materials and producing a finished product. When you load a webpage, the browser interprets the HTML and CSS code that’s there, then translates those into a visual display on your screen. To do this, it uses what’s called a layout engine, or rendering engine. The layout engine is where things get really interesting when it comes to javascript.

The engine is built to understand how the HTML and CSS code it’s running will change the page, and then executes a piece of javascript to make that happen. To do this, the engine creates something known as a Call Stack, which is a LIFO data structure that manages function calls. As a function is invoked, its stack frame is pushed onto the stack, and then the execution will resume at the return address specified in that stack frame.

To demonstrate how this works, let’s look at a small example of javascript that takes a number and calculates the square of it. To run this, you’ll need a modern browser that supports javascript. Open a webpage and click the three dots in the top right corner to open the menu and then select More Tools > Developer Tools or press Control+Shift+J (Windows) or Command+Option+J (Mac). This opens up the JavaScript Console, where you can enter javascript and see it executed immediately.

There are loads of different libraries for javascript that can take care of the more complicated and time-consuming bits of creating dynamic web applications. Some of the most popular ones include jQuery, Prototype, and Dojo Toolkit. These libraries help developers build complex web applications much more quickly than they would be able to do by writing the code themselves. Having these libraries in place means that a lot of the grunt work is already done and you can focus on building your own unique application.