Rails spinoffs - Jan 2008 - Event.observe() throwing error when element not on page

Hi guys,

In my DOM ready login (Event.observe(window, ''load'',
function()
etc...) I am setting up my page observers. This is working great
except certain elements aren''t available on all pages so their causing
Javascript errors when the observer is looking for them and they
aren''t there.

Currently I''m getting around this by doing the following in my DOM
ready script:

$(''frm'') === null ? "" :
Event.observe(''frm'', ''submit'', handler);

Now while this works I''m not sure if it follows "best
practise" for
Prototype.

Thoughts?
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On Jan 22, 2008 11:13 PM, Michael Sharman
<sharmo-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> This is working great
> except certain elements aren''t available on all pages so their
causing
You should not be attempting to attach events to elements that are not
on the page, Prototype is not a "fail silently" framework.

I usually do something like your above code, except I would fetch the
element once to avoid multiple $() calls, such as:

var elem = $(''frm'');
if (elem) elem.observe(''submit'', handler);

-justin

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Hi Justin,
>You should not be attempting to attach events to elements that are not on
the page
The code is in an external .js file and there are only a couple of
pages where that element doesn''t appear.
>var elem = $(''frm'');
I guess I can''t do that though as $(''frm'')
won''t exist and throw an
error. Remember that this code exists in the ''onload'' code
block.



On Jan 23, 4:49 pm, "Justin Perkins"
<justinperk...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> On Jan 22, 2008 11:13 PM, Michael Sharman
<sha...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>
> > This is working great
> > except certain elements aren''t available on all pages so
their causing
>
> You should not be attempting to attach events to elements that are not
> on the page, Prototype is not a "fail silently" framework.
>
> I usually do something like your above code, except I would fetch the
> element once to avoid multiple $() calls, such as:
>
> var elem = $(''frm'');
> if (elem) elem.observe(''submit'', handler);
>
> -justin
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Michael Sharman wrote:
> Hi Justin,
>
>   
>> You should not be attempting to attach events to elements that are not
on the page
>>     
>
> The code is in an external .js file and there are only a couple of
> pages where that element doesn''t appear.
>
>   
>> var elem = $(''frm'');
>>     
>
> I guess I can''t do that though as $(''frm'')
won''t exist and throw an
> error. Remember that this code exists in the ''onload''
code block.
>   
Why CAN''T you do it?  If you need to keep the error console clean, use
a
try-catch block.

try { $(''frm'').observe(''submit'', handler); }
catch (e) {}


- Ken Snyder

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I may misunderstand your question, but you can always wait until the
window loads:

Event.observe(window, ''load'',
  function()
  {
    Event.observe(''frm'', ''submit'', handler);
  }
);

On Jan 23, 10:16 am, Ken Snyder
<kendsny...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> Michael Sharman wrote:
> > Hi Justin,
>
> >> You should not be attempting to attach events to elements that are
not on the page
>
> > The code is in an external .js file and there are only a couple of
> > pages where that element doesn''t appear.
>
> >> var elem = $(''frm'');
>
> > I guess I can''t do that though as $(''frm'')
won''t exist and throw an
> > error. Remember that this code exists in the
''onload'' code block.
>
> Why CAN''T you do it?  If you need to keep the error console clean,
use a
> try-catch block.
>
> try { $(''frm'').observe(''submit'',
handler); } catch (e) {}
>
> - Ken Snyder
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Michael,

"var elem = $(''frm'');" will not throw error if
element with id of
"frm" does not exist. It will assign null to "elem"
variable.

Justin''s example will work as expected and is one of the preferred
ways to check for element existence.

Alternatively, Element.observe(''frm'',
''submit'', handler) will fail
*silently* if element is not present.

I would avoid using "try ... catch" as it happens to be quite
expensive (speed wise)


Hope this helps.

Best,
kangax

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Hi kangax (and Justin),

Thanks for that, makes sense now I have morning clarity!

Appreciate your time :)

On Jan 24, 7:51 am, kangax
<kan...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> Michael,
>
> "var elem = $(''frm'');" will not throw error if
element with id of
> "frm" does not exist. It will assign null to "elem"
variable.
>
> Justin''s example will work as expected and is one of the preferred
> ways to check for element existence.
>
> Alternatively, Element.observe(''frm'',
''submit'', handler) will fail
> *silently* if element is not present.
>
> I would avoid using "try ... catch" as it happens to be quite
> expensive (speed wise)
>
> Hope this helps.
>
> Best,
> kangax
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