页面checkbox处理

function checkbox(obj) {
var checkbox = document.getElementsByName("checkbox");
var change = document.getElementById("change");
var deletebutton = document.getElementById("delete");
var download = document.getElementById("download");
for (var i=0;i<checkbox.length;i++){
checkbox[i].checked = obj.checked;
}
if (checkbox.length != 0) {
if (obj.checked == true) {
deletebutton.disabled = false;
download.disabled = false;
change.disabled = false;
} else {
deletebutton.disabled = true;
download.disabled = true;
change.disabled = true;
}
}
}

function chlidbox() {
var checkbox = document.getElementsByName("checkbox");
var mainbox = document.getElementById("mainbox");
var change = document.getElementById("change");
var deletebutton = document.getElementById("delete");
var download = document.getElementById("download");
var checknum = 0;
var unchecknum = 0;

for (var i=0;i<checkbox.length;i++){
if (checkbox[i].checked == true){
checknum++;
deletebutton.disabled = false;
download.disabled = false;
change.disabled = false;
} else if (checkbox[i].checked == false){
unchecknum++;
}
}
if (checknum == checkbox.length){
mainbox.checked = true;
} else if (unchecknum == checkbox.length){
deletebutton.disabled = true;
download.disabled = true;
change.disabled = true;
mainbox.checked = false;
} else {
mainbox.checked = false;
}

}