include jquery

JavaScript
<head>

 <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.4.1/jquery.min.js"></script>

</head>
 <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.4.1/jquery.min.js"></script><!--You can insert Jquery into HTML with the <script> tag in the <head>-->
<!--Insert the next line of code into your <head> tags.-->
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.4.1/jquery.min.js"></script> <!-- jquery (help with javascript) --><head>

<script src="https://ajax.aspnetcdn.com/ajax/jQuery/jquery-3.4.1.min.js"></script>

</head>
 <!-- First either download, or use CDN and reference it -->
<!-- Here we grab it from a CDN -->
<script
  src="https://code.jquery.com/jquery-3.4.1.js"
  integrity="sha256-WpOohJOqMqqyKL9FccASB9O0KwACQJpFTUBLTYOVvVU="
  crossorigin="anonymous"></script>

<!-- Lets say we have some <p>, with simple text 
	 we give it an id -->
<p id="example">Hello, world</p>

<!-- We shall now play around with this element using jQuery -->
<script type="text/javascript">
	// The vanilla way of accessing our example p element is
  	var e = document.getElementById('example');
  	// Then we would faff about with 'e'
  
  	// With jQuery we can get it like...
  	var ej = $('#example');
  
  	// jQuery has its own methods, instead of innerText and
  	// all those similar properties, we can do
  	$('#example').text("Goodbye everybody, I've got to go");
  
  	// Our example p element text will have changed :)
</script><!--You can insert Jquery into HTML with the <script> tag.-->
<head>
  <script type = 'text/javascript' src = 'https://code.jquery.com/jquery-3.3.1.js'></script>
</head>
Source