How can I use JavaScript to create a basic keylogger for ethical hacking purposes

Question

I’m exploring ethical hacking techniques and I’ve heard that keyloggers can be useful for demonstrating vulnerabilities in insecure systems. I’m interested in creating a basic keylogger using JavaScript on my own test environment.

How can I use JavaScript to capture keystrokes from a web page? Are there any legal and ethical considerations I should be aware of when experimenting with this type of script? I want to ensure I’m only using it in a controlled, ethical manner during security research or penetration testing.