JSX unfamiliar syntax. Where to read?

Question

React

Explain to me the old man what kind of syntax is this,

const routes = ( /* тут понаписано */) 
// или вот такое 
let foobar = ( /* и тут понаписано  */)

I look at the description https://developer.mozilla.org/en-US/docs/Web/Java... and see that this is not possible.
What is it and where does it come from? Where can you read?
Well, in addition, I stumbled upon something like "@withStyles" in one application with react. What is it?

Answer 1

Good morning, this is ES6 .
habrahabr.ru/post/175371