全选与全不选

<input type="checkbox" );

var y = true
a.onchange = function(){
if(a.checked){
for(var i = 0 ; i<=b.length;i++)
b[i].checked = true;
}else{
for(var i = 0 ; i<=b.length;i++){
b[i].checked = false;
}
}
}

</script>