History of JavaScript dialogs
JavaScript was introduced in 1995, and in the very first version of JavaScript
were methods on the window object named
alert(),
confirm(),
and prompt().
While they fit into the JavaScript of the time, their synchronous API is problematic for modern browsers. Because the JavaScript engine needs to pause until a user response is obtained, the JavaScript dialogs are app-modal. And because the dialogs are app-modal, they commonly (and unfortunately) are used to harm our users.
Because of this, the Chromium team highly recommends that you not use JavaScript dialogs.
Alternatives
There are many options for dialog replacement.
There are several choices for alert()/confirm()/prompt(). For notifying the
user of events (e.g. calendaring sites), the
Notifications API
should be used. For obtaining user input, the
HTML <dialog> element
should be used. For XSS proofs-of-concept, devtool’s
console.log(document.origin) can be used.
As for onbeforeunload, it should be noted that it is already unreliable. As
Ilya Grigorik points out,
“You cannot rely on pagehide, beforeunload, and unload events to fire on
mobile platforms.” If you need to save state, you should use the
Page Visibility API.
Changes
The ability for a page to specify the onbeforeunload string was
removed in Chrome 51.
(It was also removed by Safari starting with Safari 9.1 and in Firefox 4.)
alert()/confirm()/prompt() dialogs have changed from being app-modal to being dismissed when their tab is switched from. This
change took place across all channels at the beginning of May 2017.
beforeunload dialogs require a user gesture on the page to allow them to show
starting in Chrome 60.
(This does not change the dispatching of the beforeunload event.) This aligns
Chromium with Firefox, which made this change with Firefox 44.
Showing an alert()/confirm()/prompt() dialog while in fullscreen will cause
fullscreen to be lost
starting in Chrome 61.
prompt() dialogs do not activate their tab. If prompt() is called from a
background tab, the call returns immediately and no dialog is shown.
This change
took place across all channels at the beginning of May 2017.
alert() dialogs do not activate their tab. If alert() is called from a
background tab, the call returns immediately. The tab is marked with an indicator
and the user will see the dialog when they switch to the tab. This behavior change
is seen
starting in Chrome 64.
confirm() dialogs do not activate their tab. If confirm() is called from a
background tab, the call returns immediately and no dialog is shown.
This behavior change is seen
starting in Chrome 69.
Because of these changes, if your site uses dialogs, it is highly recommended that you move to using the earlier-mentioned alternatives so that this will not affect you.