index.js

import React from 'react'
import ReactDOM from 'react-dom'
import { Router, browserHistory, applyRouterMiddleware } from 'react-router'
import Routes from './routes'
import Relay from 'react-relay'
import useRelay from 'react-router-relay'
import { RelayNetworkLayer, urlMiddleware } from 'react-relay-network-layer'
import { relayApi } from './config/endpoints'
import auth from './utils/auth' 

const createHeaders = () => {
  let idToken = auth.getToken()
  if (idToken) {
    return {
      'Authorization': `Bearer ${idToken}`
    }
  } else {
    return {}
  }
}

Relay.injectNetworkLayer(
  new RelayNetworkLayer([
    urlMiddleware({
      url: (req) => relayApi,
    }),
    next => req => {
      req.headers = {
        ...req.headers,
        ...createHeaders()
      }
      return next(req)
    }
  ])
)

ReactDOM.render(
  <Router
    environment={Relay.Store}
    render={applyRouterMiddleware(useRelay)}
    history={browserHistory}
    routes={Routes}
  />  
  , document.getElementById('root'))