How can i detect if the page is exited but not reloaded? [duplicate]

This question already has an answer here:

• Check if page gets reloaded or refreshed in Javascript 8 answers

I am making a Chat in HTML, JS and PHP and I'm trying to send automatically a leave message.

This should somehow look like this:

function onPageExit(){
  if(!isJustReloading){
     //do something...
  }
}