Why confirmation pop-up doesn't show the message I've defined for it?

Question

Here is my code:

window.onbeforeunload = function(evt) {
    var el = document.getElementById("qandatextarea");
    if( el && el.value && !DontAskBeforeExit){
        var message='ding ding';
        return message;
    }
}

But it shows this pop-up:

Why? I never told it this message:

Changes you made may not be saved.

Well where it comes from? And why it doesn't show the message I've defined for it? ( dib ding )

---

The result of this is also the same as the above one:

window.onbeforeunload = function(evt) {
    var el = document.getElementById("qandatextarea");
    if( el && el.value && !DontAskBeforeExit){
        var message = 'ding ding';
        if (typeof evt == 'undefined') {
            evt = window.event;
        }
        if (evt) { 
            evt.returnValue = message;
        }

        return message;
    }
}

Answer 1

This functionality has been removed from several major browsers. See the compatibility section on MDN . Notice that there is a section for "Custom text support removed". Chrome 51 and FF 44 have removed the ability to customize the text.