在ES6中Minimax实现TicTacToe没有返回适当的结果

Question

我已经为玩tic-tac-toe编写了一个minimax实现。这是一个独立的函数，接受游戏状态并转向并返回最佳位置。

以下是代码：

function minimax(grid,isMaximizingPlayer,depth){
 let turnOf = (isMaximizingPlayer)?'O':'X';
 console.log("Playing turn of : "+turnOf);
 console.log("Starting minimax @ depth: "+depth);
 let vacancies = getVacantPlaces(grid);
 let winner = calculateWinner(grid);
 if(vacancies.length === 0){
   if((isMaximizingPlayer && winner==='O') || (!isMaximizingPlayer && 
                    winner==='X'))
      return {score: 10,position: -1};
   else if((isMaximizingPlayer && winner==='X') || (!isMaximizingPlayer && 
                    winner==='O'))
      return {score: -10,position: -1};
   else if(null===winner)
      return {score: 0,position: -1};
 }
 let moves = [];
 for(let i=0;i<vacancies.length;i++){
 let move = {position: -1,score: 0};
 grid[vacancies[i]] = (isMaximizingPlayer) ? 'O' : 'X';
 move.score = minimax(grid,!isMaximizingPlayer,depth+1).score;
 if((isMaximizingPlayer && 10===move.score) ||
  (!isMaximizingPlayer && -10===move.score)){
  move.position = vacancies[i];
  console.log("Pruning other choices, best score got @: 
             "+JSON.stringify(move));
  return move;
}
//reset the grid
 grid[vacancies[i]] = null;
 moves.push(move);
}
let bestMove = {position: -1,score: 0};
if(isMaximizingPlayer){
   //start with minimum score and select max of moves
   bestMove.score = -10;
   bestMove = moves.reduce((previousBest,currentMove) => {
     return (currentMove.score > previousBest.score) ? currentMove : 
                                     previousBest;
   },bestMove);
}else {
   //start with maximum score and select min of moves
   bestMove.score = 10;
   bestMove = moves.reduce((previousBest,currentMove) => {
     return (currentMove.score < previousBest.score) ? currentMove : 
             previousBest;
   },bestMove);
}
console.log("Best move for iteration : "+ JSON.stringify(bestMove)); 
return bestMove;
}

//Testing input
var gameState = [null,null,'X',
              'O',null,'X',
              null,null,null];

console.log(JSON.stringify(minimax(gameState,true,1)));

//OUTPUTS: {"position":7,"score":10}
//EXPECTED: {"position":8,"score":0}

Here's上面代码的jsfiddle以及辅助函数，用于获取空位和计算胜利者。虽然很难但却无法找出缺失的部分。