<!DOCTYPE html>
<html>
<head>
<style>
table, th, td {
    border: 1px solid black;
}

</style>
</head>
<body>

<p>Click the button to return the value of the colspan attribute of th with id "myTh".</p>

<table>
  <tr>
    <th id="myTh" colspan="2">Month</th>
    <th>Savings</th>
  </tr>
  <tr>
    <td>January</td>
    <td>$100</td>
  </tr>
  <tr>
    <td>February</td>
    <td>$100</td>
  </tr>
</table>
<br>

<button onclick="myFunction()">Try it</button>

<p id="demo"></p>

<script>
function myFunction(x) {
    var x = document.getElementById("myTh").colSpan;
    document.getElementById("demo").innerHTML = "The value of colspan is: " + x;
}
</script>

</body>
</html>