You can't write a book on web applications without covering JavaScript. It is everywhere.

If you write a web app that does a full page load on every request and it's not a simple content site, then it will feel slow. However, users expect responsiveness.

If you are a backend developer, then you may think that you don't have to worry about this. However, if you are building an API, then you may want to make it easy to consume with JavaScript, and you will need to make sure that your JSON is correctly and quickly serialized.

Even if you are building a Single-Page Application (SPA) in JavaScript (or TypeScript) that runs in the browser, the server can still play a key role. You can use SPA services to run Angular or React on the server and generate the initial output. This can increase performance as the browser has something to render immediately. For example, there is a project called React.NET that integrates React with ASP.NET, and it supports ASP.NET Core.

If you have been struggling to keep up with the latest developments in the .NET world, then JavaScript is on another level. There seems to be something new almost every week, and this can lead to framework fatigue and a paradox of choice. There is so much to choose from that you don't know what to pick.

We will cover some of the more modern practices later in the book and show the improved performance that they can bring. We'll look at service workers and show how they can be used to move work into the background of a browser to make it feel more responsive to the user.