How to call a function before leaving page with Javascript

Question

I would like to execute a function before leaving page without showing a confirmation popup with Javascript only. I've tried with the code below but it didn't work or with the onbeforeunload but it always shows the popup.

var result = 'test';

if(window.onbeforeunload == true)
{
    result = 'test1';
    alertmess();
}

function alertmess() {
    alert(result);
}

//window.onbeforeunload = function() { 
//  return result; 
//}

Answer 1

You can always call your function before leaving the page.

function myfun(){      // Write your business logic here      console.log('hello'); } 

onbeforeunload:

window.onbeforeunload = function(){   myfun();   return 'Are you sure you want to leave?'; }; 

Or with jQuery:

$(window).bind('beforeunload', function(){   myfun();   return 'Are you sure you want to leave?'; }); 

This will just ask the user if they want to leave the page or not, you cannot redirect them if they select to stay on the page. If they select to leave, the browser will go where they told it to go.

You can use onunload to do stuff before the page is unloaded, but you cannot redirect from there (Chrome 14+ blocks alerts inside onunload):

window.onunload = function() {     myfun();     alert('Bye.'); } 

Or with jQuery:

$(window).unload(function(){   myfun();   alert('Bye.'); }); 

Answer 2

Just call your function from within window.onbeforeunload. Note, some browsers restrict what you can do here (eg: no redirects or alerts). See: https://developer.mozilla.org/en-US/docs/Web/Events/beforeunload for more info.

I've also added the appropriate code for readers that do want to show a confirmation dialog.

function doSomething(){
    //do some stuff here. eg:
    document.getElementById("test").innerHTML="Goodbye!";
}
function showADialog(e){
    var confirmationMessage = 'Your message here';
    //some of the older browsers require you to set the return value of the event
    (e || window.event).returnValue = confirmationMessage;     // Gecko and Trident
    return confirmationMessage;                                // Gecko and WebKit
}
window.addEventListener("beforeunload", function (e) {
    //To do something (Remember, redirects or alerts are blocked here by most browsers):
    doSomething();    
    //To show a dialog (uncomment to test):
    //return showADialog(e);  
});

Just hit 'Run' to test: http://jsfiddle.net/2Lv4pa9p/1/