turn = (turn==false);

var game_board[3][3]; // this is your game board
var winner = 0; // the winner of this game. 0=nobody, 1=player1, 2=player2, etc.

function check_across( row, player )
{
   // check if the player wins
   if( game_board[row][0] == player )
   if( game_board[row][1] == player )
   if( game_board[row][2] == player )
   {
     // the player who placed all these identical pieces in a row wins!
     winner = player;
   }
}

function check_down( column, player )
{
   // check if the player wins
   if( game_board[0][column] == player )
   if( game_board[1][column] == player )
   if( game_board[2][column] == player )
   {
     // the player who placed all these identical pieces in a column wins!
     winner = player;
   }
}

function check_diagonals( player )
{
   // check if the player wins down and right
   if( game_board[0][0] == player )
   if( game_board[1][1] == player )
   if( game_board[2][2] == player )
   {
     // the player who placed all these identical pieces down and right wins!
     winner = player;
   }

   // check if the player wins down and left
   if( game_board[0][2] == player )
   if( game_board[1][1] == player )
   if( game_board[2][0] == player )
   {
     // the player who placed all these identical pieces down and left wins!
     winner = player;
   }
}

function end_of_turn( player )
{
   var c;

   // this function checks to see if a player won the game
   for( c=0; c<3; c++ )
   {
      check_across( c, player );
      check_down( c, player );
   }
   check_diagonals( player );

   // did this player win?
   if( winner == player )
   {
      // celebrate his winning, restart the game, etc. etc.
   }
}