Update a textarea with RJS

Hello

I would like a update a texteara with JS. with this simple code. It's
working well

RJS
page["objectif"].value = "test"

View
<%= f.text_area :objectif, :rows => 10 , :cols => 55 , :id =>
"objectif"%>

But when i try to test this code
page["objectif"].value = page["textarea"].value

  <select id="textara" name="textara" style="margin-left: 30px;">
    <option value="objectif">Objectfis</option>
  </select>
  <a href="#" id="add_doc">
    <img src="../images/add.gif" alt="Ajouter un document" style="border:
0; margin-left: 50px; vertical-align: middle;">
  </a>
</div>
<h3>Objectifs :</h3><%= f.text_area :objectif, :rows => 10 , :cols =>
55 , :id => "objectif"%>

No working. Can you help me

Hello

i was testing this

@value =
page["textarea"].options[page["textarea"].selectedIndex].value

i have an error

[Break on this error] $
("textarea").selectedIndex().#<ActionView::Helpers::JavaScriptElementProxy:
0x30...