Tryit Editor v2.9

Edit This Code:
<!DOCTYPE html>
<html>
<head>
<style>
table, td {
    border: 1px solid black;
}
</style>
</head>
<body>

<p>Click the button to insert new cell(s) at the beginning of the first table row.</p>

<table id="myTable">
  <tr>
    <td>First cell</td>
    <td>Second cell</td>
    <td>Third cell</td>
  </tr>
  <tr>
    <td>First cell</td>
    <td>Second cell</td>
    <td>Third cell</td>
  </tr>
</table><br>

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

<script>
function myFunction() {
    var firstRow = document.getElementById("myTable").rows[0];
    var x = firstRow.insertCell(0);
    x.innerHTML = "New cell";
}
</script>

</body>
</html>