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 ...
