<script>

function displayResult(){

    alert(document.getElementById("myTable").rows[0].cells.length);

}

</script>

<table id="myTable" border="1">

    <tr>

        <td>cell 1</td>

        <td>cell 2</td>

    </tr>

    <tr>

        <td>cell 3</td>

        <td>cell 4</td>

    </tr>

</table>

<br>

<button type="button" onclick="displayResult()">显示第一行单元格的数量</button>

​