# RJS, Prototype and finding out if object exists in the DOM

**URL:** <https://www.ruby-forum.com/t/rjs-prototype-and-finding-out-if-object-exists-in-the-dom/88668>\
**Category:** Rails\
**Created:** [March 13, 2007, 1:17pm UTC](https://www.ruby-forum.com/t/rjs-prototype-and-finding-out-if-object-exists-in-the-dom/88668 "2007-03-13T13:17:28Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![zecko](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/z/9de053/32.png) [@zecko](https://www.ruby-forum.com/u/zecko)\
**Post date:** [March 13, 2007, 1:17pm UTC](https://www.ruby-forum.com/t/rjs-prototype-and-finding-out-if-object-exists-in-the-dom/88668/1 "2007-03-13T13:17:28Z")

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i asked this allready in german on [rubyforum.com](http://rubyforum.com) and also on  
#rubyonrails (thanks again for your time, njero)  
[http://forum.ruby-portal.de/viewtopic.php?t=4067](http://forum.ruby-portal.de/viewtopic.php?t=4067)

how is it possible to find out, if an object (div) exists in the DOM via  
RJS? i am trying to find out if an object exists before applying a  
visual action on it, so there is no javascript error produced.  
generally, objects can be fetched via getElementById, which is the same  
as $(‘elementid’) in prototype. but this seems to be useless, since it  
is always creating an ActionView::Helpers::JavaScriptVariableProxy - so  
i cannot check if the object _exists_

here is the two ways i tried

1. RJS and Prototype

# the rjs

thename = ‘person’+params[‘id’].to\_s  
if page[thename] then  
p “YES”  
else  
p “NO”  
end

# this is in the html

 ![](https://www.ruby-forum.com/images/icons/person_f.png)

1. Custom Javascript

thename = ‘person’+params[‘id’].to\_s  
page \<\< (“function existing(invar){ if (document.getElementById(invar)  
!= null) {return true; }else {return false;} }”)  
r=page.call :existing,thename

if r then p “yes”  
else p “no”  
end

any more ideas how to solve that problem? i am wondering why nobody  
seems to have had this problem allready. how do you guys prevent  
javascript errors? is there another way to have sort of safe RJS calls?

thanks!  
zecko

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**Author:** ![zecko](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/z/9de053/32.png) [@zecko](https://www.ruby-forum.com/u/zecko)\
**Post date:** [March 13, 2007, 2:38pm UTC](https://www.ruby-forum.com/t/rjs-prototype-and-finding-out-if-object-exists-in-the-dom/88668/2 "2007-03-13T14:38:17Z")

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Hi Moe,

Moe Zecko wrote:

> how is it possible to find out, if an object (div) exists  
> in the DOM via RJS? i am trying to find out if an  
> object exists before applying a visual action on it, so  
> there is no javascript error produced.

Interesting question. I’ll start by saying I still consider myself  
pretty  
new to Rails, js, and Ajax. I was out of hands-on development for 15  
years  
and have only been working with Rails for a little over a year now. So  
I  
either don’t understand your question, or your context, or both 😉  
More  
below.

> i am wondering why nobody seems to have had  
> this problem allready. how do you guys prevent  
> javascript errors? is there another way to have  
> sort of safe RJS calls?

I think Rails probably eliminates most of the need for the kind of thing  
you’re asking about. In general, I know that something exists in the  
DOM  
because I put it there. 99.9% of the time, the page content in a Rails  
app  
is rendered dynamically. If it’s on the page, it’s there because I just  
put  
it there; typically as a result of something that’s stored in the  
database.  
In your case, it looks like you’re probably doing or could be doing a  
find  
on your Person model. If the find returns a result, then there’s a DOM  
element. If the situation were one where the Person record could exist  
in  
the database but I might or might not have rendered a DOM element, I’d  
either put a field in the database or use a session variable to track  
it,  
depending on the situation.

hth,  
Bill

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<div class="post-metadata">

**Author:** ![zecko](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/z/9de053/32.png) [@zecko](https://www.ruby-forum.com/u/zecko)\
**Post date:** [March 13, 2007, 2:44pm UTC](https://www.ruby-forum.com/t/rjs-prototype-and-finding-out-if-object-exists-in-the-dom/88668/3 "2007-03-13T14:44:18Z")

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The only way I’ve found to do IF statements in RJS is to write them in  
traditional JS, eg:

page \<\<  
“if( $(element) ){  
Element.remove(element);  
}”

Someone wrote a patch for RJS to include IF statements, but I don’t  
believe  
it was included. Search Trac for the code if you’re interested.

ed

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**Author:** ![zecko](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/z/9de053/32.png) [@zecko](https://www.ruby-forum.com/u/zecko)\
**Post date:** [March 13, 2007, 3:28pm UTC](https://www.ruby-forum.com/t/rjs-prototype-and-finding-out-if-object-exists-in-the-dom/88668/4 "2007-03-13T15:28:38Z")

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Hi Bill,

> I think Rails probably eliminates most of the need for the kind of thing  
> you’re asking about. In general, I know that something exists in the  
> DOM because I put it there. 99.9% of the time, the page content in a Rails  
> app is rendered dynamically. If it’s on the page, it’s there because I just  
> put it there; typically as a result of something that’s stored in the  
> database.

the problem is the following: i know that the entry exists in the  
database, but i don’t know if the representation of that item is  
currently displayed on the screen.  
there are 2 parts on the screen: on the left side is a search result  
(titles only), on the right side there is the detail view for a specific  
entry. if i am deleting an entry on the right side, it should also  
remove the representation, if it is there. since i don’t know if it is  
there (the user might have changed the search-result via ajax-search) i  
have to check if that specific object exists before removing it. else  
there is a javascript-error popup appearing and javascript might not  
work for future actions.  
so there are 2 ways to solve this problem:

1. check if the object is existing in the left side-bar
2. run some sort of “safe-rjs-action”, which does not fail if the object  
is not existing.

for both ways i did not succeed in finding any answers.

thanks for any hints,  
mz.

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<div class="post-metadata">

**Author:** ![zecko](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/z/9de053/32.png) [@zecko](https://www.ruby-forum.com/u/zecko)\
**Post date:** [March 13, 2007, 4:37pm UTC](https://www.ruby-forum.com/t/rjs-prototype-and-finding-out-if-object-exists-in-the-dom/88668/5 "2007-03-13T16:37:43Z")

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Hi Moe,

Moe Zecko wrote:

> work for future actions.  
> so there are 2 ways to solve this problem:
> 
> 1. check if the object is existing in the left side-bar
> 2. run some sort of “safe-rjs-action”, which does not fail if the object  
> is not existing.

I see three ways, but to start I’d back up to your problem statement.  
The  
reason you don’t know if the item is in the left side is because you’re  
allowing the two sides to get disconnected. What does the visitor see  
on  
the right side immediately after their _first_ search? Is it blank (or  
some  
default content) until they click one of the items on the left to get  
details? Or is it the first item? Or something else? Whatever it is,  
my  
recommendation would be to treat every search the same way. Otherwise  
you’re setting yourself up for a problem re: consistent-user-experience.  
So…

1. Every time the user does a search, change the right side to whatever  
default you choose.

If you insist on letting the disconnect occur…

1. Every time the user does a search, persist the results and whatever  
else  
you need to identify the DOM elements that’ll be on the left side in a  
session variable. When the user deletes an item from the right side, in  
your delete\_right\_side method, check the persisted search results to see  
if  
the item is in there. If it is, delete the DOM element from the left.

Note that (2) still leaves you with a ‘?’. Independent of whether or  
not  
there’s a corresponding item on the left, what’s going to show up on the  
right side when they delete an item from there?

1. Don’t use RJS for this. Like Rails, it’s not intended to be a  
be-all-end-all. It’s one tool that’s very useful in the circumstances  
it  
was intended to address. This may or may not be one of those  
circumstances.

hth,  
Bill