how to know the old value of an element onChange or onTrigger in JQuery?

how to know the old value of an element onChange or onTrigger in JQuery?

HTML

<button id="btn1">test_2</button>

JavaScript

$(document).ready(function()
{
    var i=1;
    $('#btn1').click(function()
    {
    		$(this).data('old', $(this).text()).text('btn'+(++i));
        alert( $(this).data('old'));
    });
});