全选和全不选checkboxList

var checkflag = "false";
function check(field) {
if (checkflag == "false") {
for (i = 0; i < field.length; i++) {
field[i].checked = true;}
checkflag = "true";
return "false"; }
else {
for (i = 0; i < field.length; i++) {
field[i].checked = false; }
checkflag = "false";
return "true"; }
}