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

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();
        }
    });
});