diagonal.js

        ///     | j j j j j
        ///     | 0 1 2 3 4 < j-index
        /// ----+----------- round-th
        /// i 0 | 0 1 2 3 4  round-0: [0,0]
        /// i 1 | 1 2 3 4 5  round-1: [0,1][1,0]
        /// i 2 | 2 3 4 5 6  round-2: [0,2][1,1][2,0]
        /// i 3 | 3 4 5 6 7  round-3: [0,3][1,2][2,1][3,0]
        /// i 4 | 4 5 6 7 8  round-4: [0,4][1,3][2,2][3,1][4,0]
        ///                  round-5:      [1,4][2,3][3,2][4,1]
        ///                  round-6:           [2,4][3,3][4,2]
        ///                  round-7:                [3,4][4,3]
        ///                  round-8:                     [4,4]
        /// [0,1,2],
        /// [3,4,5],
        /// [6,7,8],
        /// travelR([[0,1,2],[3,4,5],[6,7,8]]).forEach(([it, j]) => { console.log(it[j]); });
        /// 0,3,1,6,4,2,7,5,8
        ///
        /// [0],
        /// [3,4,5],
        /// [6,7,8],
        /// 0,3,6,4,7,5,8
        /// travelR([[0],[3,4,5],[6,7,8]]).forEach(([it, j]) => { console.log(it[j]); });
        /// [0],
        /// [3,4,5],
        /// [6],
        /// travelR([[0],[3,4,5],[6]]).forEach(([it, j]) => { console.log(it[j]); });
        /// 0,3,6,4,5
        function* diagonal(matrix) {
            const rows = matrix.length;
            const cols = Math.max(...matrix.map(it => it.length));
        
            // Traverse from top-left to bottom-right
            for (let r = 0; r < rows + cols - 1; r++) {
                let i = Math.min(r, rows - 1);
                let j = r - i;
        
                while (i >= 0 && j < cols) {
                    const row = matrix[i];
                    console.log(`diagonal: ${i},${j}: ${row[j]}`);
                    if (j < row.length) yield [row, j];
                    i--;
                    j++;
                }
            }
        }