getWinnerFromTicTacToe.ts

enum SquarePossibilities {
  X = 'X',
  O = 'O',
}

type SquareType = SquarePossibilities | null;

/** 
 * Return winner from tic tac toe game
 * @argument squares: SquareType[]
 * @returns SquareType
*/
function getWinnerFromTicTacToe(squares: SquareType[]): SquareType {
  const winCombinations = [
    [0, 1, 2],
    [3, 4, 5],
    [6, 7, 8],
    [0, 3, 6],
    [1, 4, 7],
    [2, 5, 8],
    [0, 4, 8],
    [2, 4, 6],
  ];

  for (const combination of winCombinations) {
    const [a, b, c] = combination;
    if (
      squares[a]
      && squares[a] === squares[b]
      && squares[a] === squares[c]
    ) {
      return squares[a];
    }
  }

  return null;
}