<!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>