Try running this example and you'll get a nice popup with the world-famous message to the world. As we talked about previously, this tutorial will start without focusing on the browser, but I do want to mention that if you embed this code in an HTML file, it will have to be inside a SCRIPT tag, like this:
<script> alert("Hello, world!"); </script>
<!DOCTYPE html> <html> <head> <title>Hello, world!</title> </head> <body> <p>And now for the world famous message:</p> <script> document.write("Hello, world!"); </script> <p>Thank you, and goodnight!</p> </body> </html>
Try running this example, either directly here in the article, or by saving it as an .html file on your computer and opening it in your browser.
If you run this in the browser, you will have to look in the developer console to read the message. How you access this area depends on which browser you use, but you can try pressing F12 - if that doesn't work, you may want to check out this article, which explains how you access it in various browsers.