context-store.js

const http = require('http');
const fetch = require('node-fetch');
const async_hooks = require('async_hooks');

const context = new Map();

const middleware = (req, res) => {
    const id = async_hooks.executionAsyncId();
    const headers = req.headers;
    context.set(id, headers);
    res.on('finish', () => context.delete(id));
};

const getHeadersFromContext = () => {
    return context.get(async_hooks.executionAsyncId());
};

http.createServer((req, res) => {
    middleware(req, res);

    //route handler of some sort
    handleReq(req, res);

}).listen(5000);


function handleReq(req, res) {
    // Somewhere else in app
    const headers = getHeadersFromContext();

    fetch('http://localhost:3000', {
        headers: { 'User-Agent': headers['user-agent'] },
    }).then(r => {
        res.end('hello\n');
    })
}