Untitled

var btnStart = document.getElementById('mec-book-form-btn-step-1');

if (btnStart) {
    btnStart.addEventListener('onclick', function (e) {
        var btnNext = document.getElementById('mec-book-form-btn-step-2');

        btnNext.addEventListener('onclick', function (e) {
            var oib = document.getElementById('mec_field_book_text_1').value;
            if (oib.length !== 11) {
                alert('OIB treba sadržavati 11 znakova.');

                if (event.stopPropagation) {
                    event.stopPropagation();
                } else {
                    event.cancelBubble = true;
                }
            }
        });
    });
}