Check the winner of a Tic Tac Toe game

ticTacToeWinner.ts

export class InvalidBoardError extends Error {}

export default function ticTacToeWinner<T extends number | string>(
  board: T[][],
): T | "Tie" {
  // Check if the board is a valid game board
  for (let i = 0; i < board.length; i++) {
    if (board.length !== board[i].length) {
      throw new InvalidBoardError();
    }
  }

  const edge = board.length;

  // Check rows
  parent: for (let i = 0; i < edge; i++) {
    const char = board[i][0];

    for (let j = 1; j < edge; j++) {
      if (board[i][j] !== board[i][j - 1]) {
        continue parent;
      }
    }

    return char;
  }

  // Check columns
  parent: for (let i = 1; i < edge; i++) {
    const char = board[i][0];

    for (let j = 0; j < edge; j++) {
      if (board[i][j] !== board[i - 1][j]) {
        continue parent;
      }
    }

    return char;
  }

  // Check diagonals
  for (let i = 0; i < 2; i++) {
    for (let j = 1; j < edge; j++) {
      const char = board[0][0];

      if (board[j][j] !== board[j - 1][j - 1]) {
        break;
      }

      if (j === edge - 1) {
        return char;
      }
    }

    // Reverse the board for checking other diagonal
    board = board.map((item) => item.reverse());
  }

  return "Tie";
}