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javascript-我想用JavaScript改變td的colspan的值.為什麼無法改變?

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我想用JavaScript改變td的colspan的值.為什麼無法改變?

 <!DOCTYPE html>
<html>
<head>
<script type="text/javascript">
    var colspanNum=0;
    window.onload=function(){
        var thisTable = document.getElementById("thisTable");
        var trs = thisTable.getElementsByTagName("tr");
        for(var i=1;i<trs.length;i++){
            var tds = trs[i].getElementsByTagName("td");
            if(i==1){
                var tdLength = tds.length;
                colspanNum = tdLength;
                var spanButton = document.getElementById("spanButton");
                spanButton.colspan=colspanNum;
                console.log(colspanNum);
            }
            for(var j=0;j<tds.length;j++){
                if(j==1){
                    console.log(tds[j].innerHTML);
                }
            }
        }
    }
    //console.log(colspanNum);
</script>
</head>
<body>
    <table id="thisTable">
        <tr>
            <td>id</td>
            <td>name</td>
            <td>age</td>
        </tr>
        <tr>
            <td>001</td>
            <td>Laowang</td>
            <td>10</td>
        </tr>
        <tr>
            <td>002</td>
            <td>Yan</td>
            <td>9</td>
        </tr>
        <tr>
            <td id="spanButton"><button>button</button></td>
        </tr>
    </table>
</body>
</html>