“SelectAll” checkbox not working when back button on the browser is clicked

<html>
<body>

<script>
    var check = 0;

function doNow()
{
  void(d=document);

  void(el=d.getElementsByTagName('INPUT'));
  for(i=0;i<el.length;i++)
    {
        if(check == 0)
            void(el[i].checked=1)
        else
            void(el[i].checked=0)
    }
   if(check == 0)
       check = 1;
   else
       check = 0;
}

</script>
<HR>
<form>
<INPUT TYPE="CHECKBOX" name="DG1">DG1
<INPUT TYPE="CHECKBOX" name="DG2">DG2
<INPUT TYPE="CHECKBOX" name="DG3">DG3
<INPUT TYPE="CHECKBOX" name="DG4">DG4
<INPUT TYPE="CHECKBOX" name="DG5">DG5

<input type=button onclick="doNow()" value="CheckBox All Now">

</form>
</body>
</html>

var check = 0;

var check = document.getElementsByTagName('INPUT')[0].checked;

if(check == 0)
    void(el[i].checked=1)
else
    void(el[i].checked=0)

el[i].checked = (1 - check);