How to add td after a td

I have a td and I want to add a new one, but my new td is shown as text on my page, what am I missing
my code :

        var firstTd = parent.document.getElementById("currentTd");           
        firstTd.after('<td><span>teste</span></td>');

>Solution :

See the documentation:

The Element.after() method inserts a set of Node or string objects in the children list of the Element’s parent, just after the Element. String objects are inserted as equivalent Text nodes.

So pass an element instead.

const firstTd = parent.document.getElementById("currentTd");           
const secondTd = document.createElement('td');
const span = document.createElement('span');
span.textContent = "teste";
secondTd.appendChild(span);
firstTd.after(secondTd);

or consider insertAdjacentHTML