javascript check checkbox programmatically

1// Check
2document.getElementById("checkbox").checked = true;
3
4// Uncheck
5document.getElementById("checkbox").checked = false;

1<script type="text/javascript">
2    function selectAll() {
3        var items = document.getElementsByName('acs');
4        for (var i = 0; i < items.length; i++) {
5            if (items[i].type == 'checkbox')
6                items[i].checked = true;
7        }
8    }
9
10    function UnSelectAll() {
11        var items = document.getElementsByName('acs');
12        for (var i = 0; i < items.length; i++) {
13            if (items[i].type == 'checkbox')
14                items[i].checked = false;
15        }
16    }			
17</script>
18