CheckBox 如何实现全选？_JavaScript

在一个表单中有很多CheckBox，请问如何实现选种一个而让所有的都选中？  
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function  selectAll(checkBoxName){  
           var  elms  =  document.getElementsByName(checkBoxName);  
           if  (elms.length           var  allChecked  =  true;  
           for  (var  i=0;i                       if  (elms[i].type=="checkbox")  
                                   allChecked  &=  elms[i].checked;  
           for  (var  i=0;i                       if  (elms[i].type=="checkbox"){  
                                               elms[i].checked  =  !allChecked;  
                                               if  (elms[i].onclick)  elms[i].onclick();  
                       }  
}  
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 全选    

<script> <BR>function mm(o) <BR>{ <BR> var a = document.getElementsByName("m"); <BR> for (var i=0; i<a.length; i++){ <BR> a[i].checked = o.checked; <BR> } <br><br>} <BR></script>









全选：
<script> <BR> function sel(a){ <BR> o=document.getElementsByName(a) <BR> for(i=0;i<o.length;i++) <BR> o[i].checked=event.srcElement.checked <BR> } <BR></script>