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Modal prompts

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Revision as of 13:17, 16 February 2012 by Jochen (talk | contribs)
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This document describes a proposal for gradually removing modal dialogs and prompts from the platform.


In a tabbed browser, modal dialogs are potentially disrupting the work flow of the user as they can't interact with any other web site as long as the modal dialog is displayed.

Furthermore, implementing showModalDialog might require running a nested message loop which caused several security issues in the past.

Issues with current implementations

Chromium for example doesn't display a window created by showModalDialog in a modal way: http://crbug.com/16045

WebKit and Firefox don't suppress events while a modal dialog is running: https://bugs.webkit.org/show_bug.cgi?id=78240 and https://bugzilla.mozilla.org/show_bug.cgi?id=360872

Firefox displays modal prompts as tab-modal. However, it's possible to execute JavaScript in a tab that should be blocked by a modal prompt: https://bugzilla.mozilla.org/show_bug.cgi?id=727397 and the prompts from separate tabs can block each other: https://bugzilla.mozilla.org/show_bug.cgi?id=727802



For the following modal prompts, a callback parameter is introduced

  • alert
  • confirm
  • prompt

When these functions are invoked with a callback parameter, they return immediately (instead of blocking). The callback is invoked as soon as the modal dialog closes. For confirm and prompt, the callback takes one parameter which is set to the value to modal version would have returned.


Current interface

 void alert(DOMString message);
 boolean confirm(DOMString message);
 DOMString? prompt(DOMString message, optional DOMString default);

Proposed interface

 void alert(DOMString message, optional VoidCallback complete);
 boolean confirm(DOMString message, BooleanCallback complete);
 DOMString? prompt(DOMString message, optional BooleanCallback complete);
 DOMString? prompt(DOMString message, DOMString default, optional BooleanCallback complete);

prompt requires overloading, as the existing interface already has an optional parameter.


I would like to propose deprecating and eventually removing window.showModalDialog from the platform. Web sites can already now use e.g. window.open instead. However, I'm not quite sure how this could be achieved.


window.confirm("Are you sure?", function(result) { if (result) doIt(); });


  • What would a reasonable plan to deprecate showModalDialog look like?
  • Does it make sense to add a callback to alert at all?
  • Should we rather invest in <dialog> and then try to deprecate all of the modal prompts and dialogs?