What does the highlighted line that starts with if do?

Question

JavaScript

//Запускает таймер
    function go(){
      window.timerId = window.setInterval(timer, 1000);
    }
    function timer(){
      if(window.number == undefined || window.number == 3) {
      window.number = 1;
      } else window.number = window.number + 1;
      var img = document.getElementById('img');
      img.src = 'smiles/'+window.number+'.png'
    }

Answer 1

I think it should be understood like this:
If the property "window.number" does not exist or is equal to 3 ,
then set this property to 1 ,
otherwise add 1 to the existing value .

Answer 2

Loops the counter of pictures, and also initiates it in case of absence. The latter, by the way, is bad practice.