I see that most modern browsers have removed the ability to pass custom message in onbeforeunload dialogs. (feature removed in chrome: https://www.chromestatus.com/feature/5349061406228480).
So i have the following questions..
Why have the modern browsers removed that functionality ? What is their reasoning behind this design? If user has to show custom messages in modern browsers , what do we do now?
The reason behind that , is security concerns . As onbeforeunload event may allow attackers to upload some codes which can merge harsh code to client / server sides as well as to the uploaded file.