JS a checkboxy

	function mycheck(){
document.getElementsByValue("a").checked='checked';
}
</script>

<form name="formular">
<input type="checkbox" value="a" id="1" name="a" onchange="mycheck('a');">
<input type="checkbox" value="b" id="2" name="b" onchange="mycheck('b');">
<input type="checkbox" value="a" id="3"  name="c" onchange="mycheck('a');">
</form>

<script>
function SelectAllFromGroup( state, gID ) {
    var group = document.getElementById( gID );
    var inputs = group.getElementsByTagName( 'input' );
    for ( var i = 0; i < inputs.length; i++ ) {
        if ( inputs[i].type == 'checkbox' ) {
            inputs[i].checked = state;
        }
    }
}

</script>
<input type="checkbox" onclick="SelectAllFromGroup(this.checked, 'group1');"/> Group 1
<div id="group1">
    <input type="checkbox"/> Item 1.1
    <input type="checkbox"/> Item 1.2
    <input type="checkbox"/> Item 1.3
    <input type="checkbox"/> Item 1.4
    <input type="checkbox"/> Item 1.5
    <input type="checkbox"/> Item 1.6
</div>