<html>
<head>
<script>
function displayResult()
{
alert(document.getElementById("myTable").rows.length);
}
</script>
</head>
<body>
<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()">Show number of rows in table</button>
</body>
</html>