TicTacToe.js

const prompt = require('prompt');

class TicTacToe {

  constructor() {
    this.winner = null;
    this.board = [
      [' ', ' ', ' '],
      [' ', ' ', ' '],
      [' ', ' ', ' '],
    ];
    this.turn = 'X';
  }

  checkWinner() {
    const board = this.board;
    if (board[0][1] === board[0][0] && board[0][2] === board[0][0]) {
      this.winner = board[0][0];
    } else if (board[1][1] === board[1][0] && board[1][2] === board[1][0]) {
      this.winner = board[1][0];
    } else if (board[2][1] === board[2][0] && board[2][2] === board[2][0]) {
      this.winner = board[2][0];
    } else if (board[0][0] === board[1][0] && board[0][0] === board[2][0]) {
      this.winner = board[0][0];
    } else if (board[0][1] === board[1][1] && board[0][1] === board[2][1]) {
      this.winner = board[0][1];
    } else if (board[0][2] === board[1][2] && board[0][2] === board[2][2]) {
      this.winner = board[0][2];
    } else if (board[0][0] === board[1][1] && board[0][0] === board[2][2]) {
      this.winner = board[0][0];
    } else if (board[0][2] === board[1][1] && board[0][2] === board[2][0]) {
      this.winner = board[0][2];
    }
    return this.winner;
  }

  mark() {
    console.log(`Current Turn: ${this.turn}`);
    prompt.get(['Row', 'Col'], (err, result) => {
    if (err) { return onErr(err); }
    this.board[result.Row][result.Col] = this.turn;
    this.display();
    const currentWinner = this.checkWinner();
    if (currentWinner !== ' ') {
      console.log(`${this.winner} is the winner!`);
      return;
    }
    this.turn = (this.turn === 'X') ? 'O' : 'X';
    this.mark();
    });
  }
  display() {
    console.log(`${this.board[0][0]} | ${this.board[0][1]} | ${this.board[0][2]}`);
    console.log('--- --- ---');
    console.log(`${this.board[1][0]} | ${this.board[1][1]} | ${this.board[1][2]}`);
    console.log('--- --- ---');
    console.log(`${this.board[2][0]} | ${this.board[2][1]} | ${this.board[2][2]}`);
  }
};

prompt.start();
const game = new TicTacToe();
game.display();
game.mark();

function onErr(err) {
  console.log(err);
  return 1;
}