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var btnNext = element.getElementById('mec-book-form-btn-step-2');

if(btnNext) {
    btnNext.addEventListener('onclick', function (e) {
    var oib = element.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;
            }
        }
    });
}