Untitled

<input type="text" class="forever" />

input {

  display: block;

}

document.addEventListener("DOMContentLoaded", function () {

  var el = document.querySelector(".forever");

  el.addEventListener("input", foreverize);

});

function foreverize (evt) {

  var allEls = document.querySelectorAll(".forever");

  if(allEls[allEls.length-1].value !== "") {

    var newel = document.createElement("input");

    newel.classList.add("forever");

    newel.addEventListener("input", foreverize);

    evt.target.parentElement.appendChild(newel);

  }

}