Set input / textarea direction to rtl if the field contains RTL characters.

set-input-direction.js

  $(document).on('input', 'input, textarea', function () {
    var $el = $(this),
        val = $el.val();

    if (val.length) {
      $el.css({direction: isRTL(val) ? 'rtl' : 'ltr'});
    } else {
      $el.css({direction: ""});
    }
  });

  function isRTL(s) {
    return (/[\u0591-\u07FF\uFB1D-\uFDFD\uFE70-\uFEFC]/).test(s);
  }