用js判斷一個復選框是否被選中


<html> <head>
        <title>
            復選框全選、全不選、反選、必選一個
        </title>
        <meta http-equiv="content-type" content="text/html;charset=GBK"/>
        <script language="javascript">
            //表單驗證
            function check(){               
                var ids = document.getElementsByName("ids");               
                var flag = false ;               
                for(var i=0;i<ids.length;i++){
                    if(ids[i].checked){
                        flag = true ;
                        break ;
                    }
                }
                if(!flag){
                    alert("請最少選擇一項!");
                    return false ;
                }
            }
            //全選
            function iselect(){ //其中函數字不能為select 其為JS保留字
                var ids = document.getElementsByName("ids");
                var all = document.getElementByIdx_x_x("all");               
                for(var i=0;i<ids.length;i++){
                    ids[i].checked=all.checked;
                }
            }
            //全選
            function selectAll(){
                var ids = document.getElementsByName("ids");                           
                for(var i=0;i<ids.length;i++){
                    ids[i].checked=true;
                }
            }
            //全不
            function selectNone(){
                var ids = document.getElementsByName("ids");                           
                for(var i=0;i<ids.length;i++){
                    ids[i].checked=false;
                }
            }
            //反選
            function selectInvert(){
                var ids = document.getElementsByName("ids");                           
                for(var i=0;i<ids.length;i++){
                    if(ids[i].checked)
                        ids[i].checked=false ;
                    else
                        ids[i].checked=true ;
                }
            }
        </script>
    </head>
    <body>
        <form action="#" onsubmit="return check();">
        <table border="0">
            <tr><td><input type="checkbox" id="all" onclick="iselect();"></td></tr>
            <tr><td><input type="checkbox" name="ids">音樂</td></tr>
            <tr><td><input type="checkbox" name="ids">短信</td></tr>
            <tr><td><input type="checkbox" name="ids">圖片</td></tr>
            <tr><td><input type="checkbox" name="ids">電子書</td></tr>
            <tr><td><input type="checkbox" name="ids">鈴聲</td></tr>
            <tr><td><input type="checkbox" name="ids">漫畫</td></tr>
            <tr><td><input type="checkbox" name="ids">游戲</td></tr>
            <tr><td><input type="checkbox" name="ids">雜志</td></tr>
            <tr><td><input type="checkbox" name="ids">新聞</td></tr>
            <tr>
                <td>
                    <a href="#" onclick="selectAll();">全選</a>&nbsp;
                    <a href="#" onclick="selectNone();">全不</a>&nbsp;
                    <a href="#" onclick="selectInvert();">反選</a>
                </td>
            </tr>
            <tr><td><input type="submit" value="提交"></td></tr>
        </table>
        </form>
    </body>
</html>


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