When a popup is blocked, window.open() returns undefined instead of a reference to the newly created window. This is why you see undefined after the axios.post.

document.getElementById('myButton').addEventListener('click', () => {
  axios.post(url, formData, { headers: { 'Content-Type': 'application/form-data' } })
    .then(res => {
      window.open("https://javascript.info");
    });
});