just wondering if anyone knows anything of using javascript to set html to new content instead of linking to new pages, if this is generally a bad idea or if it kind of hurts SEO(which im kind of new to)

Basically the home page displays given content, and the links to like contact pages and stuff, just change the body content to what would normally be a separate html page. my OCD kinda bugs me when pages reload and either flash the background or its offset somehow, so i wanted to know if making sites like this was a bad idea or whatever-

i suppose at the least, i could create duplicates/hidden pages for SEO purposes