on top of the site you cant use the React js implementation via adobe target. But you can use the Adobe target API to check the whhich variant are you in based on that you can process feature in application itself.
helpful, specifically: that according to the Target Product Documentation on at.js Implementation , although at.js communicates with the Target servers asynchronously, the at.js file itself must load synchronously in the <head> section of your page. Ideally, it should be one of the first scripts loaded. Once loaded, at.js executes mbox calls asynchronously through XMLHttpRequest, and does not block page rendering." Hope this is helpful - feel free to reach out here again if you have more questions!