Javascript is a scripting language, which can be run at the client side. Which mainly uses to manipulate the DOM(Document Object Model).
But as of today, JavaScript can execute not only in the browser, but also on the server, or actually on any device that has a special program called the JavaScript engine.

JavaScript programs can be used almost anywhere into an HTML document using the <script> tag.
Lets see a typical example of Hello World.

<!DOCTYPE HTML>
<html>
<body>
  <p>Script Start</p>
  <script>
    alert( 'Hello, world!' );
  </script>
  <p>Script End</p>
</body>
</html>


 For more information visit https://javascript.info