How to show the "Are you sure you want to navigate away from this page?" when changes committed?

Update (2017)

Modern browsers now consider displaying a custom message to be a security hazard and it has therefore been removed from all of them. Browsers now only display generic messages. Since we no longer have to worry about setting the message, it is as simple as:

// Enable navigation prompt
window.onbeforeunload = function() {
    return true;
};
// Remove navigation prompt
window.onbeforeunload = null;

Read below for legacy browser support.

Update (2013)

The orginal answer is suitable for IE6-8 and FX1-3.5 (which is what we were targeting back in 2009 when it was written), but is rather out of date now and won't work in most current browsers - I've left it below for reference.

The window.onbeforeunload is not treated consistently by all browsers. It should be a function reference and not a string (as the original answer stated) but that will work in older browsers because the check for most of them appears to be whether anything is assigned to onbeforeunload (including a function that returns null).

You set window.onbeforeunload to a function reference, but in older browsers you have to set the returnValue of the event instead of just returning a string:

var confirmOnPageExit = function (e) 
{
    // If we haven't been passed the event get the window.event
    e = e || window.event;

    var message = 'Any text will block the navigation and display a prompt';

    // For IE6-8 and Firefox prior to version 4
    if (e) 
    {
        e.returnValue = message;
    }

    // For Chrome, Safari, IE8+ and Opera 12+
    return message;
};

You can't have that confirmOnPageExit do the check and return null if you want the user to continue without the message. You still need to remove the event to reliably turn it on and off:

// Turn it on - assign the function that returns the string
window.onbeforeunload = confirmOnPageExit;

// Turn it off - remove the function entirely
window.onbeforeunload = null;

Original answer (worked in 2009)

To turn it on:

window.onbeforeunload = "Are you sure you want to leave?";

To turn it off:

window.onbeforeunload = null;

Bear in mind that this isn't a normal event - you can't bind to it in the standard way.

To check for values? That depends on your validation framework.

In jQuery this could be something like (very basic example):

$('input').change(function() {
    if( $(this).val() != "" )
        window.onbeforeunload = "Are you sure you want to leave?";
});

The onbeforeunload Microsoft-ism is the closest thing we have to a standard solution, but be aware that browser support is uneven; e.g. for Opera it only works in version 12 and later (still in beta as of this writing).

Also, for maximum compatibility, you need to do more than simply return a string, as explained on the Mozilla Developer Network.

Example: Define the following two functions for enabling/disabling the navigation prompt (cf. the MDN example):

function enableBeforeUnload() {
    window.onbeforeunload = function (e) {
        return "Discard changes?";
    };
}
function disableBeforeUnload() {
    window.onbeforeunload = null;
}

Then define a form like this:

<form method="POST" action="" onsubmit="disableBeforeUnload();">
    <textarea name="text"
              onchange="enableBeforeUnload();"
              onkeyup="enableBeforeUnload();">
    </textarea>
    <button type="submit">Save</button>
</form>

This way, the user will only be warned about navigating away if he has changed the text area, and will not be prompted when he's actually submitting the form.

To make this work in Chrome and Safari, you would have to do it like this

window.onbeforeunload = function(e) {
    return "Sure you want to leave?";
};

Reference: https://developer.mozilla.org/en/DOM/window.onbeforeunload