js-tetris - rotate

board.js

// Transpose matrix, p is the Piece.
for (let y = 0; y < p.shape.length; ++y) {
  for (let x = 0; x < y; ++x) {
    [p.shape[x][y], p.shape[y][x]] = 
    [p.shape[y][x], p.shape[x][y]];
  }
}

// Reverse the order of the columns.
p.shape.forEach(row => row.reverse());