app.get('/play/:cell/:color', (req, res) => {
    // TODO: security
    let cell = req.params['cell'];
    let color = req.params['color'];
    game.grid[cell] = color;
    if (game.state.next == 'X') {
        game.state.next = 'O';
    } else {
        game.state.next = 'X';
    }
    res.json(game);
});