The this
statement in surroundtest
applies to the window
object and not the element. What you should do is to change the function definition as so:
function surroundtest(element, text2,text3){
var c = $(element).parent().next('textarea');
...
}
And the HTML accordingly:
<div class="guides_chapters_button" onclick="surroundtest(this, '[center]', '[/center]');">Center</div>