用js判断一个复选框是否被选中

       
       
        <script language="javascript">
            //表单验证
            function check(){               
                var ids = document.getElementsByName("ids");               
                var flag = false ;               
                for(var i=0;i                    if(ids[i].checked){
                        flag = true ;
                        break ;
                    }
                }
                if(!flag){
                    alert("请最少选择一项！");
                    return false ;
                }
            }
            //全选
            function iselect(){ //其中函数字不能为select 其为JS保留字
                var ids = document.getElementsByName("ids");
                var all = document.getElementByIdx_x_x("all");               
                for(var i=0;i                    ids[i].checked=all.checked;
                }
            }
            //全选
            function selectAll(){
                var ids = document.getElementsByName("ids");                           
                for(var i=0;i                    ids[i].checked=true;
                }
            }
            //全不
            function selectNone(){
                var ids = document.getElementsByName("ids");                           
                for(var i=0;i                    ids[i].checked=false;
                }
            }
            //反选
            function selectInvert(){
                var ids = document.getElementsByName("ids");                           
                for(var i=0;i                    if(ids[i].checked)
                        ids[i].checked=false ;
                    else
                        ids[i].checked=true ;
                }
            }
        </script>