//Store the status of the checkboxes
var checkflag = false;

/**
 * Check al checkboxes with the specified name
 */
function check(name) {
  //get all checkboxes
  var checkboxes = document.getElementsByName(name)
  //invert value of checkflag
  checkflag = !checkflag;
  //invert value on each checkbox
  for (i = 0; i < checkboxes.length; i++) {
    checkboxes[i].checked = checkflag;
  }
}