text caret position

HTML

<input id="txt" value="some text here" />
<input type="button" id="btn" value="OK" />

CSS

#txt {
    width: 300px;
}

JavaScript

jQuery("#btn").on('click', function() {
    var caretPos = document.getElementById("txt").selectionStart;
    var textAreaTxt = jQuery("#txt").val();
    var decindex = textAreaTxt.indexOf(.);
    var txtToAdd = "more text";
    //console.log(caretPos);
    alert(caretPos);
    alert(decindex);
    jQuery("#txt").val(textAreaTxt.substring(0, caretPos) + txtToAdd + textAreaTxt.substring(caretPos) );
});