JS winner checker function

gameFinished.js

function gameFinished() {
      // check rows
      for (i = 1; i < 4; i++)
        if (document.getElementById('div_'+i+'1').innerHTML == document.getElementById('div_'+i+'2').innerHTML &&
            document.getElementById('div_'+i+'2').innerHTML == document.getElementById('div_'+i+'2').innerHTML)
          return document.getElementById('div_'+i+'1').innerHTML;

      // check cols
      for (i = 1; i < 4; i++)
        if (document.getElementById('div_1'+i).innerHTML == document.getElementById('div_2'+i).innerHTML &&
            document.getElementById('div_2'+i).innerHTML == document.getElementById('div_3'+i).innerHTML)
          return document.getElementById('div_1'+i).innerHTML;

      // check main diag
      if (document.getElementById('div_11').innerHTML == document.getElementById('div_22').innerHTML &&
          document.getElementById('div_22').innerHTML == document.getElementById('div_33').innerHTML)
        return document.getElementById('div_22').innerHTML;

      // check secondary diag
      if (document.getElementById('div_13').innerHTML == document.getElementById('div_22').innerHTML &&
          document.getElementById('div_22').innerHTML == document.getElementById('div_31').innerHTML)
        return document.getElementById('div_22').innerHTML;

      return null;
    }