App.js

import React, { useState, useEffect } from "react";
import axios from "axios";

const App = () => {
  const [username, setUsername] = useState("");
  const [password, setPassword] = useState("");
  const [user, setUser] = useState();

  useEffect(() => {
    const loggedInUser = localStorage.getItem("user");
    if (loggedInUser) {
      const foundUser = JSON.parse(loggedInUser);
      setUser(foundUser);
    }
  }, []);

  // logout the user
  const handleLogout = () => {
    setUser({});
    setUsername("");
    setPassword("");
    localStorage.clear();
  };

  // login the user
  const handleSubmit = async e => {
    e.preventDefault();
    const user = { username, password };
    // send the username and password to the server
    const response = await axios.post(
      "http://blogservice.herokuapp.com/api/login",
      user
    );
    // set the state of the user
    setUser(response.data);
    // store the user in localStorage
    localStorage.setItem("user", JSON.stringify(response.data));
  };

  // if there's a user show the message below
  if (user) {
    return (
      <div>
        {user.name} is loggged in
        <button onClick={handleLogout}>logout</button>
      </div>
    );
  }

  // if there's no user, show the login form
  return (
    <div>
      <form onSubmit={handleSubmit}>
        <label htmlFor="username">Username: </label>
        <input
          type="text"
          value={username}
          placeholder="enter a username"
          onChange={({ target }) => setUsername(target.value)}
        />
        <div>
          <label htmlFor="password">password: </label>
          <input
            type="password"
            value={password}
            placeholder="enter a password"
            onChange={({ target }) => setPassword(target.value)}
          />
        </div>
        <button type="submit">Login</button>
      </form>
    </div>
  );
};

export default App;

Storing the entire user information instead of just a token is really dangerous because the user information can be easily hacked. Additionally, it doesn't make sense to use JSON Web Tokens (jsonwebtoken) if you are not using them on the client-side as well. This tutorial is useless.