Trouble incrementing object property - TicTacToe Project

let startHTML = '<div class="screen screen-start" id="start"><header><h1>Tic Tac Toe</h1><a href="#" class="button">Start game</a></header></div>';
let startScreen;
let startButton;
let player1;
let player2;
let game1;

let body = document.getElementsByTagName('body')[0];

/*------------------- DOMContentLoaded Listener -------------------*/
document.addEventListener('DOMContentLoaded', () => {
    body.innerHTML += startHTML;
    startScreen = document.getElementsByClassName('screen-start')[0];
    startButton = document.getElementsByClassName('button')[0];

//event listener which creates the players and hides the start screen
    startButton.addEventListener('click',()=>{

        player1 = new Player (prompt('PLAYER ONE: Enter your name.'));

        player2 = new Player (prompt('PLAYER Two: Enter your name.'));

        player1.card = document.getElementById('player1');
        player2.card = document.getElementById('player2');

        $(startScreen).hide();

        game1 = new Game (player1, player2);

        game1.startGame();
    });

});

/*------------------- Constructor Functions -------------------*/
function Player (name){
    this.name = name;
    this.score = 0;
    this.card;
};

function Game (player1, player2){
    this.player1 = player1;
    this.player2 = player2;
    this.turnIndex = 0;
};
/*------------------- Prototype Functions -------------------*/
Game.prototype.nextTurn = () => {
    $(player1.card).toggleClass('active');
    $(player2.card).toggleClass('active');
    this.turnIndex += 1;
};

Game.prototype.startGame = () => { 
    this.turnIndex = 1;
    if(Math.floor(Math.random()*2 <=1)){
            console.log('starting game');
            $(player1.card).addClass('active');
        } else {
            $(player2.card).addClass('active');
        }
};