Let's say you are implementing user authetication and authorization in a MERN stack app using JWTs. Once the user registration or login is successful, you save the JWT received from the server in localStorage.

Now, in order to access protected resources, you must send the JWT on the Authorization request header.

We can send a JWT on every request using Axios config defaults, as shown in the highlighted code snippet below:

import React, { useEffect } from "react"
import { Routes, Route } from "react-router-dom"
import axios from "axios"
import Header from "./components/Header"
import Home from "./components/Home"
import Register from "./components/Register"
import Login from "./components/Login"
import NewTicket from "./components/NewTicket"
import Tickets from "./components/Tickets"
import SingleTicket from "./components/SingleTicket"
import PrivateRoute from "./components/PrivateRoute"
import jwt_decode from "jwt-decode"
import { useDispatch } from "react-redux"
import { updateName } from "./components/authSlice"

const App = () => {
  const dispatch = useDispatch()

  useEffect(() => {
    const tokenObj = localStorage.getItem("authStatus")
      ? JSON.parse(localStorage.getItem("authStatus"))
      : null

    if (tokenObj) {
      const { token } = tokenObj
      axios.defaults.headers.common["Authorization"] = `Bearer ${token}`
      const { name } = jwt_decode(token)
      dispatch(updateName(name))
    }
  }, [dispatch])
  return (
    <>
      <Header />
      <Routes>
        <Route path="/" element={<Home />} />
        <Route path="/register" element={<Register />} />
        <Route path="/tickets/:id" element={<SingleTicket />} />
        <Route path="/login" element={<Login />} />
        <Route path="/new-ticket" element={<PrivateRoute />}>
          <Route path="/new-ticket" element={<NewTicket />} />
        </Route>
        <Route path="/tickets" element={<PrivateRoute />}>
          <Route path="/tickets" element={<Tickets />} />
        </Route>
      </Routes>
    </>
  )
}

export default App

Check more details here.