Js to determine whether the checkbox is selected

Js determines whether the checkbox is selected. Example code:
<!doctype html>  
<html lang="cn">  
 <head>  
 <meta http-equiv="content-type" content="text/html;charset=UTF-8"/>  
  <title>复选框全选、全不选、反选、必选一个</title>  
   <script type="text/javascript">  
        //全选  
        function selectAll() {  
            var names = document.getElementsByName("bjjb");                             
            for(var i=0;i<names.length;i++){  
                    names[i].checked=true;  
             }        
        }  

        //全不选  
        function selectNone() {  
            var names = document.getElementsByName("bjjb");                             
            for(var i=0;i<names.length;i++){  
                    names[i].checked=false;  
             }        
        }  

         //反选  
        function selectInvert() {  
            var names = document.getElementsByName("bjjb");                                 
            for(var i=0;i<names.length;i++){  
                    if(names[i].checked){  
                        names[i].checked=false ;  
                    }else{  
                        names[i].checked=true ;  
                    }  
                }  
        }  

         //必须选择一个  
        function selectOne() {  
            var names = document.getElementsByName("bjjb");            
            var flag = false ;//标记判断是否选中一个                 
            for(var i=0;i<names.length;i++){  
                if(names[i].checked){  
                        flag = true ;  
                        break ;  
                 }  
             }  
             if(!flag){  
                alert("请最少选择一项！");  
                return false ;  
             }  
        }  

   </script>  
 </head>  
 <body>  
    <form id="checkboxform" method="post" action="">  
        <input type="checkbox" name="bjjb" value="1">交通事故<br>  
        <input type="checkbox" name="bjjb" value="2">自然灾害<br>  
        <input type="checkbox" name="bjjb" value="3">恶劣天气<br>  
        <input type="checkbox" name="bjjb" value="4">严重违法行为<br>  
        <input type="checkbox" name="bjjb" value="5">路面损毁<br>  
        <a href="javascript:void(0);"  onclick="selectAll()">全选</a>   
        <a href="javascript:void(0);" onclick="selectNone()">全不选</a>   
        <a href="javascript:void(0);" onclick="selectInvert()">反选</a>   
        <a href="javascript:void(0);" onclick="selectOne()">必须选择一个</a>   
     </form>  
 </body>  
</html>
Js to determine whether the checkbox is selected

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