test.js

/*
lets visualize the above game state
                                   O |   | X
                                   ---------
                                   X |   | X
                                   ---------
                                     | O | O
                             //       ||        \\
                O | X | X          O |   | X        O |   | X
                ---------          ---------        ---------
                X |   | X          X | X | X        X |   | X
                ---------          ---------        ---------
                  | O | O            | O | O        X | O | O
              //          \\                     //          \\
        O | X | X          O | X | X        O | O | X       O |   | X
        ---------          ---------        ---------       ---------
        X | O | X          X |   | X        X |   | X       X | O | X
        ---------          ---------        ---------       ---------
          | O | O          O | O | O        X | O | O       X | O | O
                                        //
                                   O | O | X
                                   ---------
                                   X | X | X
                                   ---------
                                   O | O | O
*/
// human
var huPlayer = "O";
// ai
var aiPlayer = "X";

// this is the board flattened and filled with some values to easier asses the Artificial Inteligence.
var origBoard = [0,1 ,"X","X",4 ,"X", 6 ,"O","O"];
//var origBoard = [0,1 ,2,3,4 ,5, 6 ,7,8];

//keeps count of function calls
var fc = 0;

// finding the ultimate play on the game that favors the computer
var bestSpot = minimax(origBoard, aiPlayer);

//loging the results
console.log("index: " + bestSpot.index);
console.log("function calls: " + fc);

// the main minimax function
function minimax(newBoard, player){
  //add one to function calls
  fc++;

  //available spots
  var availSpots = emptyIndexies(newBoard);

  // checks for the terminal states such as win, lose, and tie and returning a value accordingly
  if (winning(newBoard, huPlayer)){
     return {score:-10};
  }
	else if (winning(newBoard, aiPlayer)){
    return {score:10};
	}
  else if (availSpots.length === 0){
  	return {score:0};
  }

// an array to collect all the objects
  var moves = [];

  // loop through available spots
  for (var i = 0; i < availSpots.length; i++){
    //create an object for each and store the index of that spot that was stored as a number in the object's index key
    var move = {};
  	move.index = newBoard[availSpots[i]];

    // set the empty spot to the current player
    newBoard[availSpots[i]] = player;

    //if collect the score resulted from calling minimax on the opponent of the current player
    if (player == aiPlayer){
      var result = minimax(newBoard, huPlayer);
      move.score = result.score;
    }
    else{
      var result = minimax(newBoard, aiPlayer);
      move.score = result.score;
    }

    //reset the spot to empty
    newBoard[availSpots[i]] = move.index;

    // push the object to the array
    moves.push(move);
  }

// if it is the computer's turn loop over the moves and choose the move with the highest score
  var bestMove;
  if(player === aiPlayer){
    var bestScore = -10000;
    for(var i = 0; i < moves.length; i++){
      if(moves[i].score > bestScore){
        bestScore = moves[i].score;
        bestMove = i;
      }
    }
  }else{

// else loop over the moves and choose the move with the lowest score
    var bestScore = 10000;
    for(var i = 0; i < moves.length; i++){
      if(moves[i].score < bestScore){
        bestScore = moves[i].score;
        bestMove = i;
      }
    }
  }

// return the chosen move (object) from the array to the higher depth
  return moves[bestMove];
}

// returns the available spots on the board
function emptyIndexies(board){
  return  board.filter(s => s != "O" && s != "X");
}

// winning combinations using the board indexies for instace the first win could be 3 xes in a row
function winning(board, player){
 if (
        (board[0] == player && board[1] == player && board[2] == player) ||
        (board[3] == player && board[4] == player && board[5] == player) ||
        (board[6] == player && board[7] == player && board[8] == player) ||
        (board[0] == player && board[3] == player && board[6] == player) ||
        (board[1] == player && board[4] == player && board[7] == player) ||
        (board[2] == player && board[5] == player && board[8] == player) ||
        (board[0] == player && board[4] == player && board[8] == player) ||
        (board[2] == player && board[4] == player && board[6] == player)
        ) {
        return true;
    } else {
        return false;
    }
}