const WINNING_LINES = [
  [0,1,2],[3,4,5],[6,7,8], // rows
  [0,3,6],[1,4,7],[2,5,8], // cols
  [0,4,8],[2,4,6],          // diagonals
]

function getWinner(board) {
  for (const [a, b, c] of WINNING_LINES) {
    if (board[a] && board[a] === board[b]
                 && board[a] === board[c])
      return board[a]
  }
  return null
}