what are the advantages of react js

Over what? Generally compared to normal websites it will let you create reusable components and create interactive websites faster than let's say jquery. It's also supposed to help you create large projects faster. Also you can make single page apps, but you'll have to make an api to properly consume data or use something like Inertiajs.

Good mouthfeel.

There's a lot of advantages like extremely slow First Contentful Paint times, massive amounts of transmitted JavaScript, horrible user experience

Definitely would recommend 👍

it's not brainfuck, i guess.

It eats your memory

It is trendy and shiny, like an Apple product!