Q:

React Router BrowserRouter leads to "404 Not Found - nginx " error when going to subpage directly without through a home-page click

I am using React Router for routing for a multi-page website.  When trying to go to a sub page directly https://test0809.herokuapp.com/signin you'd get a "404 Not Found -nginx" error (To be able to see this problem you might need to go to this link in Incognito mode so there's no cache).  All the links work fine if you go from the home page: test0809.herokuapp.com/. I was using BrowserRouter and was able to eliminate the "404 not found" error by changing BrowserRouter to HashRouter, which gives all my urls a "#" sign.  Besides all the problems with having a "#" in your urls, the biggest issue with it is that I need to implement LinkedIn Auth in my website, and LinkedIn OAuth 2.0 does not allow redirect URLs to contain #. 
LinedIn OAuth 2.0 error screen grab
import React, { Component } from 'react'
import { BrowserRouter as Router, Route, Link } from 'react-router-dom'
import LinkedIn from 'react-linkedin-login'
const Home = () => <div><h2>Home</h2></div>
const About = () => <div><h2>About</h2></div>
class Signin extends Component {
  callbackLinkedIn = code => {
    console.log(1, code)
  }
  render() {
      return (
          <div>
              <h2>Signin</h2>
              <LinkedIn
                  clientId="clientID"
                  callback={this.callbackLinkedIn}
              >
          </div>
      )
  }
}
const BasicExample = () =>
  <Router>
    <div>
      <ul>
         <li>
           <Link to="/">Home</Link>
         </li>
         <li>
           <Link to="/about">About</Link>
         </li>
         <li>
           <Link to="/signin">Signin</Link>
         </li>
      </ul>
  <hr />

      <Route exact path="/" component={Home} />
      <Route path="/about" component={About} />
      <Route path="/signin" component={Signin} />
    </div>
  </Router>
export default BasicExample

Any suggestions on the workarounds? 
Background: I started the project with create-react-app.  GitHub repo: /debelopumento/test0809

A:

The problem is that nginx doesn't know what to do with /signin.  You need to change your nginx config (usually in /etc/nginx/conf.d/) to serve your index.html regardless of the route.  Here is a sample nginx config that might help:
server {
  listen 80 default_server;
  server_name /var/www/example.com;

  root /var/www/example.com;
  index index.html index.htm;      

  location ~* \.(?:manifest|appcache|html?|xml|json)$ {
    expires -1;
    # access_log logs/static.log; # I don't usually include a static log
  }

  location ~* \.(?:css|js)$ {
    try_files $uri =404;
    expires 1y;
    access_log off;
    add_header Cache-Control "public";
  }

  # Any route containing a file extension (e.g. /devicesfile.js)
  location ~ ^.+\..+$ {
    try_files $uri =404;
  }

  # Any route that doesn't have a file extension (e.g. /devices)
  location / {
    try_files $uri $uri/ /index.html;
  }
}

A:

Just simply add try_files $uri $uri/ /index.html; to location / block in NGINX configuration file like this:
server {
   listen       80;
   server_name  localhost;

   location / {
       root   /build;
       index  index.html;
       try_files $uri $uri/ /index.html;
   }
}

