Code That Works On Fiddle But Doesn't Work On Local

I have this code: [URL] .... It works on fiddle but on local it does not work. What should be the problem?

Fiddle:

$('#addnewline').on('click', function (e) {
e.preventDefault();
var $textarea = $('#thetext');
$textarea.val(function () {
return $(this).val().substring(0, this.selectionstart) + "<br>" + $(this).val().substring(this.selectionstart);
});
});
<button id="addnewline">New line</button>
<br />
<textarea rows="4" cols="50" id="thetext">Some text