264525 - onbeforeunload capability not detectable

Status: RESOLVED WORKSFORME

(Core :: DOM: Events, enhancement)

Description

[rob]

User-Agent:       Mozilla/5.0 (Windows; U; Windows NT 5.1; rv:1.7.3) Gecko/20040913 Firefox/0.10.1
Build Identifier: Mozilla/5.0 (Windows; U; Windows NT 5.1; rv:1.7.3) Gecko/20040913 Firefox/0.10.1

Using object detection rather than browser sniffing, there is no way to know
that Firefox supports the onbeforeunload event. If it is not assigned a value,
typeof(window.onbeforeunload) returns "undefined" and unfortunately so does
Opera (which does not support the event). If it is assigned a value,
typeof(window.onbeforeunload) returns "String" and unfortunately so does Opera.
(IE returns "String" if it is set and "object" if it is not.)

Reproducible: Always
Steps to Reproduce:




At this point, still not wanting to do browser detection, I will need to check 
onbeforeunload before I set it, and see whether it is an object. This means I
will only be able to use my onbeforeunload in IE, and not fully use the
capabilities of Firefox.

Comment 1

[Jesse Ruderman]

javascript:window.onbeforeunload = function() { alert(5); }; alert(typeof
window.onbeforeunload)

I get "function", not "string".

Why do you need to check for onbeforeunload support before setting the
onbeforeunload function, anyway?

Comment 2

[rob]

If I express the script as an anonymous function rather than a string, I get
"function" also.  The problem is I still get the same thing from Opera.

The overall issue is, I am trying to detect whether the browser supports the
onbeforeunload event or not. I can check before or after I set it, I just need
to know if the browser is going to support it. So far, in any way I can figure
out to test it, Firefox and Opera are always reporting the same values, even
though Firefox supports it and Opera doesn't. (Try your script in Opera --
you'll still get "function", but if you leave the page, you won't get your "5".)

Comment 3

[Biju]

Many time I wish we had this, I put a suggestion a whatwg 
http://lists.whatwg.org/pipermail/whatwg-whatwg.org/2010-September/028634.html

Comment 4

[Biju]

jonas sicking recommend to implement  
  'onbeforeunload' in document.body ==> true

Comment 5

[Juriy "kangax" Zaytsev]

Dupe of #414853 ?

Comment 6

[:Ms2ger (he/him; ⌚ UTC+1/+2)]

Fixed in bug 659350.