A JavaScript function to get which line number of caret current position belongs to in textarea element. Demo: http://jsfiddle.net/heiswayi/oxax5ouq/

function.js

function getLineOfCaret(a) {
    var b = 0;
    if (a.selectionEnd) b = a.selectionEnd; else if (document.selection) {
        a.focus();
        var c = document.selection.createRange();
        if (null == c) b = 0; else {
            var d = a.createTextRange(), e = d.duplicate();
            d.moveToBookmark(c.getBookmark());
            e.setEndPoint("EndToStart", d);
            b = e.text.length;
        }
    }
    return a.value.substr(0, b).split("\n").length;
}