复选框需要全部选中，才令全选框被选中

<script>   
     let j_cbAll = document.getElementById("j_cbAll");
     let j_tbs = document.getElementById("j_tb").getElementsByTagName("input");
        j_cbAll.onclick= function(){
            // console.log(this.checked);
            for(let i=0;i<j_tbs.length;i++){
                j_tbs[i].checked=this.checked;
            }
        }
        for (let i=0;i<j_tbs.length;i++){
            j_tbs[i].onclick = function(){
                flag = true;
                for(let i = 0;i<j_tbs.length;i++){
                    if(!j_tbs[i].checked){
                        flag = false;
                    }
                }
                j_cbAll.checked = flag;
            }
        }

     </script>