Self-XSS - History and mitigation states that
Facebook and others now display a warning message when users open the web developer console
What is the HTML/Javascript code to display a warning message when the console is activated?
Devtools Detect is a library that tells you if the console is open. After linking it in your html file, you could use the following code to print something when the console is open:
window.addEventListener('devtoolschange', event => {
if(event.detail.isOpen){
console.log("Warning: Console is open");
}
});