Prevent a webpage from navigating away using JavaScript

Question

Using onunload allows you to display messages, but will not interrupt the navigation (because it is too late). However, using onbeforeunload will interrupt navigation:

window.onbeforeunload = function() {
  return "";
}

Note: An empty string is returned because newer browsers provide a message such as "Any unsaved changes will be lost" that cannot be overridden.

In older browsers you could specify the message to display in the prompt:

window.onbeforeunload = function() {
  return "Are you sure you want to navigate away?";
}

Unlike other methods presented here, this bit of code will not cause the browser to display a warning asking the user if he wants to leave; instead, it exploits the evented nature of the DOM to redirect back to the current page (and thus cancel navigation) before the browser has a chance to unload it from memory.

Since it works by short-circuiting navigation directly, it cannot be used to prevent the page from being closed; however, it can be used to disable frame-busting.

(function () {
    var location = window.document.location;

    var preventNavigation = function () {
        var originalHashValue = location.hash;

        window.setTimeout(function () {
            location.hash = 'preventNavigation' + ~~ (9999 * Math.random());
            location.hash = originalHashValue;
        }, 0);
    };

    window.addEventListener('beforeunload', preventNavigation, false);
    window.addEventListener('unload', preventNavigation, false);
})();

Disclaimer: You should never do this. If a page has frame-busting code on it, please respect the wishes of the author.

The equivalent in a more modern and browser compatible way, using modern addEventListener APIs.

window.addEventListener('beforeunload', (event) => {
  // Cancel the event as stated by the standard.
  event.preventDefault();
  // Chrome requires returnValue to be set.
  event.returnValue = '';
});

Source: https://developer.mozilla.org/en-US/docs/Web/Events/beforeunload

I ended up with this slightly different version:

var dirty = false;
window.onbeforeunload = function() {
    return dirty ? "If you leave this page you will lose your unsaved changes." : null;
}

Elsewhere I set the dirty flag to true when the form gets dirtied (or I otherwise want to prevent navigating away). This allows me to easily control whether or not the user gets the Confirm Navigation prompt.

With the text in the selected answer you see redundant prompts:

In Ayman's example by returning false you prevent the browser window/tab from closing.

window.onunload = function () {
  alert('You are trying to leave.');
  return false;
}

The equivalent to the accepted answer in jQuery 1.11:

$(window).on("beforeunload", function () {
    return "Please don't leave me!";
});

JSFiddle example

altCognito's answer used the unload event, which happens too late for JavaScript to abort the navigation.

Use onunload.

For jQuery, I think this works like so:

$(window).unload(function() { 
  alert("Unloading"); 
  return falseIfYouWantToButBeCareful();
});