Closest Previous Sibling
Get the closest sibling to the target <textarea> without using id's or classes or attributes on the textarea and spans elements.
HTML
<div>
<span>1</span>
<textarea></textarea>
<span>2</span>
<textarea></textarea>
<span>3</span>
<textarea></textarea>
</div>
JavaScript
$('div').on('keydown', 'textarea', function () {
$(this).closest($('span').text($(this).val()))
})