js全选 复选框的问题

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<form>
<input type="checkbox" id="zhz[]" onclick="Btn_click()" />1<br/>
<input type="checkbox" id="zhz[]" onclick="Btn_click()" />2<br/>
<input type="checkbox" id="zhz[]" onclick="Btn_click()" />3<br/>
<input type="checkbox" id="zhz[]" onclick="Btn_click()" />4<br/>
<input type="checkbox" id="zhz[]" onclick="Btn_click()" />5<br/>
<input type='checkbox' onclick="CheckAll(this.form.elements['zhz[]'], this.checked)" />Check/UnCheck All
</form>
<script language='javascript'>
<!--
function CheckAll(srcElem, do_check){
        if(typeof(srcElem)=='undefined') return;
        var cnt = (typeof(srcElem.length)!='undefined')? srcElem.length : 0;
        if(cnt){
                for(var i=0;i<cnt; i++)
                        srcElem[i].checked = do_check;
        }else
                srcElem.checked = do_check;
}
//-->
</script>
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