Much of today's web environment has changed dramatically – not only in terms of creating web applications but also when it comes to creating server-side applications. A frontend ecosystem that was dominated by jQuery and CSS frameworks such as Bootstrap has been replaced with reactive, fully fledged applications that could be mistaken for an application running on the desktop.

The language we write these applications in has also changed in a dramatic fashion. What was once a mess of var and scope issues has turned into a fast and easy-to-program language. JavaScript has not only changed the way we write our frontend, it has also changed the backend programming experience.

We are now able to write server-side applications in the language that we write our frontend in. JavaScript has also modernized, and possibly even popularized, the event-driven system with Node.js. We can now write code for both our frontend and backend in JavaScript and possibly even share the JavaScript files we generate between the two.

However, while the application landscape has evolved and many people have moved onto modern frameworks, such as React and Vue.js for the frontend and Express and Sails for the backend, many of these developers do not understand the inner workings. While this showcases just how simple it is to enter into the ecosystem, it also showcases how easy it is to not understand how to optimize our code bases.

This book focuses on teaching highly performant JavaScript. This not only means fast execution speeds, but also a lower memory footprint. This means that any frontend system will get to the user faster, and we will be able to start our applications that much faster. On top of this, we have many new technologies that have moved the web forward, such as Web Workers.