document ready

custom.js

// Run script once DOM content has loaded
document.addEventListener('DOMContentLoaded', () => {
  document.querySelector('h1').innerHTML = 'JavaScript DOM Content Loaded'
})

index.html

<!DOCTYPE html>
<html>

<head>
  <meta charset="UTF-8">
  <title>JavaScript Document Ready</title>
  <meta name="robots" content="noindex, nofollow">
  <script src="custom.js"></script>
</head>

<body>
  <h1>JavaScript Document Ready?</h1>
</body>

</html>