X and O marks alternation

After solving the problem of the previous lesson, you should get the following code:

javascriptlet cells = document.querySelectorAll('#field td');
start(cells);

function start(cells) {
  for (let cell of cells) {
    cell.addEventListener('click', function() {
      this.textContent = 'X';
    });
  }
}

Let's now make alternation of crosses and zeros. To do this, I propose to introduce a counter of moves:

javascriptfunction start(cells) {
  let i = 0; // the counter initial value
  
  for (let cell of cells) {
    cell.addEventListener('click', function() {
      this.textContent = 'X';
      
      i++; // increases the counter value
    });
  }
}

Having such a counter, we can easily implement the alternation of a cross and a zero: it is obvious that the X mark will appear at even counter values, and the O mark at odd ones.