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var formsToCheck = document.getElementsByClassName("mec-booking-form-container");

if(Array.isArray(formsToCheck) && formsToCheck.length > 0 ) {
    formsToCheck.forEach(element => {
            element.addEventListener('submit', function (e) {
            var oib = element.getElementById('mec_field_book_text_1').value;
            if (oib.length !== 11) {
                alert('OIB treba sadržavati 11 znakova.');
                e.preventDefault();
            }
        });
    });
}