For a while, JSX and new es6 syntax had flaky support in emacs, but there's been huge work on a lot of packages. Using emacs for JavaScript with React, ES6, and Flow (or Typescript, etc) is really easy and powerful in Emacs these days.
This is how you can work on modern web development projects with full support for tooling like JSX, Flow types, live eslint errors, automatic prettier.js formatting, and more.
Set up web-mode
web-mode provides most of the underlying functionality, so a huge shout-out to the maintainer(s) there.