Single Page Applications. The spaghetti mess of JavaScript-heavy monstrosities that replaced the web.

Single-page application. SPAs became popular once JS had matured enough to generate all the HTML for the page.

It's what people who didn't understand the separation of concerns with HTTP, HTML, CSS, JavaScript and server side were did by deciding to jam it all into JavaScript and entangle it in code complete and gang of four constructs and then call it superior, easy, efficient, well designed, robust ... It's ... It's simply wild.

HTML content? In the javascript. Style information? In the javascript. A way to negotiate network loading and resources, document structure? You guessed it. Want the back button to work again? More JavaScript. URLs to be universal? Even more!

Eventually you'll get the page to be almost as functional as it would have been by default had you not used any of it.

Read the mdn and w3c documentation guys, I promise you 99.9% of what you want is in there without reinventing it from base principles. It's not 2010 anymore

It's like a bad cook who is trying to fix a poorly made meal by making more mistakes, covering it in salt and oil, smearing honey over it, and calling it delicious.