React / JS - Get all request headers on page load

Is it possible to get request headers when the page just loads. By that i mean when page loads it sends request to f.ex (localhost:3000) , and that request includes some headers that i want to get to my application.

I tried something like this which i found in other thread:

let req = new XMLHttpRequest();
    req.open('GET', document.location, false);
    req.send(null);
    let headers = req.getAllResponseHeaders().toLowerCase();

this one gets me all the headers but from the response , i need the ones from the request , i need something like .getAllRequestHeaders