[рони]

javascript888,

<script>
        let arr = [[1, 2, 3, 4], [5, 6, 7, 8], [9]];
        function normalizeArr(arr, cols, box) {
            return Array.from(arr, (a, i) => (i = a.length) == cols ? a : (a.length = cols,a.fill(box, i, cols)));
        }
        document.write(JSON.stringify(normalizeArr(arr, 4, '-')));
    </script>