title: React Js: mountNode is not defined no-undef error
body: <p>I am newly learning the React Js. I found the example at this  <a href="https://reactjs.org/" rel="nofollow noreferrer">Link</a>. But when I tried the first code:</p>
  
  <pre><code>import React, { Component } from 'react';
  import logo from './logo.svg';
  import './App.css';
  import ReactDOM from 'react-dom';
  
  class HelloMessage extends React.Component {
    render() {
      return (
        &lt;div&gt;
          Hello {this.props.name}
        &lt;/div&gt;
      );
    }
  }
  
  ReactDOM.render(
    &lt;HelloMessage name="Taylor" /&gt;,
    mountNode
  );
  export default HelloMessage;
  </code></pre>
  
  <p>I am getting this error </p>
  
  <blockquote>
    <p>./src/App.js   Line 18:  'mountNode' is not defined  no-undef</p>
    
    <p>Search for the keywords to learn more about each error.</p>
  </blockquote>
  
  <p>I have already seen the answer at this <a href="https://stackoverflow.com/questions/23600443/uncaught-referenceerror-mountnode-is-not-defined">StackOverflow link</a>. But I'm sorry I couldn't get what is explained there. Provide me the suggestions. Thank you in advance!</p>