The Code (Cont.): HTML
https://ajax.googleapis.com/ajax/libs/jquery/3.5.1/jquery.min.js
There are several ways to start using jQuery on your web site:

  • Download the jQuery library from jQuery.com.
  • Include jQuery from a CDN, which is used by this application.

Instead of downloading the jQuery, you can include jQuery from a CDN (Content Delivery Network) such as Google by using the following command:
<scrip src="jQuery address"> </script>
https://www.gstatic.com/firebasejs/3.7.4/firebase.js
This address shows the Firebase SDK (software development kit) used by this application. Gstatic.com is a Web domain owned and used by Google for accessing the Gstatic server: a server where Google hosts its static content like CSS, JavaScript code, and images. The reason why Google hosts its content on that specific server is to reduce bandwidth usage and deliver the content faster.

onClick="addUser( ); return false;"
The result of button is shown at the paragraph “<p id='result'>[result]</p>,” but the result quickly disappears after the form is submitted, or the button is clicked. According to Stack Overflow, it happens because the submit button actually does the action, but the page is being refreshed immediately. There are some ways to fix the problem, such as adding “return false;” to onClick handler. 
<!DOCTYPE html>
<html lang="en">
 <head>
  <script src=
   "https://ajax.googleapis.com/ajax/libs/jquery/3.5.1/jquery.min.js">
  </script>
  <script src= "https://www.gstatic.com/firebasejs/3.7.4/firebase.js">
  </script>
  <title>Managing Firebase data</title>
 </head>

 <body>
  <center>Managing Firebase Data</center>
  <form id="contact">
   ID: <input type="text" size="3" id="ID" value="1" />
   Name: <input type="text" size="30" id="name" value="Poke Mon" />
   Email: <input type="email" size="30" id="email" value="poke@example.com" />
   <p id="result">[result]</p>

   <button type="submit" onClick="addUser( ); return false;">Add a user
   </button>   (required: ID, name, email)

   <button type="submit" onClick="showUser( ); return false;">Show a user
   </button>   (required: ID)

   <button type="submit" onClick="showAll( ); return false;">Show all users
   </button>

   <button type="submit" onClick="deleteUser( ); return false;">Delete a user
   </button>   (required: ID)

   <button type="submit" onClick="deleteDB( ); return false;">Delete the database
   </button>

   <button type="submit" onClick="updateUser( ); return false;">Update a user
   </button>   (required: ID)

   <button type="reset">Reset</button>
  </form>