var checkflag = "false";

function checkAllBoxes (fields) {
  if (checkflag == "false") {
    if (fields.length == undefined) {
      fields.checked = true;
    } else {
      for (i = 0; i < fields.length; i++) {
        fields[i].checked = true;
     }
    }
    checkflag = "true";
    return "Tout décocher"; 
  } else {
    if (fields.length == undefined) {
      fields.checked = false;
    } else {
      for (i = 0; i < fields.length; i++) {
        fields[i].checked = false; 
      }
    }
    checkflag = "false";
    return "Tout cocher";
  }
}