thanks, i was trying to create a string from my data rather than creating an array

    const values = [];
        options = Array.from(document.querySelectorAll('#categories option'));
        for (var i = 0; i < cat.length; i++) {
            values.push(cat[i].categoryid);
            }
            const multiSelect = document.querySelector('#categories');
            const multiSelectInstance = mdb.Select.getInstance(multiSelect);
            multiSelectInstance.setValue(values);