using DOM to make HTML work code example

Example 1: Html dom

<!-- USING EVENT HANDLER FOR  DOM HTML -->
<html lang="en">
<head>
   <meta charset="UTF-8" />
   <title>WEB222</title>
   <style> 
      body { margin: 0 18%; }
   </style>
   <script>
	 window.onload = function() { // why use window.onload?
		 var elem = document.querySelector("#myBtn");
		 elem.addEventListener( "click", displayDate );

	 }

     function displayDate() {
        document.querySelector("#demo").innerHTML = (new Date()).toLocaleString();
	 }
   </script>
</head>
<body>
  <h1>WEB222 - HTML DOM Event</h1>
  <p>Click "Show Current Time" to execute the displayDate() function.</p>
  <p id="demo"></p>

  <button id="myBtn">Show Current Time</button>
  
  <br>
  <!-- for downloading source files -->
  <p><a href="" Download>Download</a></p>
</body>
</html>

Example 2: JavaScript and HTML DOM Reference

const contacts = ['Chris:2232322', 'Sarah:3453456', 'Bill:7654322', 'Mary:9998769', 'Dianne:9384975'];
const para = document.querySelector('p');
const input = document.querySelector('input');
const btn = document.querySelector('button');

btn.addEventListener('click', function() {
  let searchName = input.value.toLowerCase();
  input.value = '';
  input.focus();
  for (let i = 0; i < contacts.length; i++) {
    let splitContact = contacts[i].split(':');
    if (splitContact[0].toLowerCase() === searchName) {
      para.textContent = splitContact[0] + '\'s number is ' + splitContact[1] + '.';
      break;
    } else {
      para.textContent = 'Contact not found.';
    }
  }
});