diff --git a/assets/js/input-duplicator.js b/assets/js/input-duplicator.ts
similarity index 52%
rename from assets/js/input-duplicator.js
rename to assets/js/input-duplicator.ts
index 2ffa89bc1..9fa05e7d0 100644
--- a/assets/js/input-duplicator.js
+++ b/assets/js/input-duplicator.ts
@@ -1,30 +1,27 @@
+import { assertNotNull } from './utils/assert';
 import { $, $$, disableEl, enableEl, removeEl } from './utils/dom';
 import { delegate, leftClick } from './utils/events';
 
-/**
- * @typedef InputDuplicatorOptions
- * @property {string} addButtonSelector
- * @property {string} fieldSelector
- * @property {string} maxInputCountSelector
- * @property {string} removeButtonSelector
- */
-
-/**
- * @param {InputDuplicatorOptions} options
- */
-function inputDuplicatorCreator({
+export interface InputDuplicatorOptions {
+  addButtonSelector: string;
+  fieldSelector: string;
+  maxInputCountSelector: string;
+  removeButtonSelector: string;
+}
+
+export function inputDuplicatorCreator({
   addButtonSelector,
   fieldSelector,
   maxInputCountSelector,
   removeButtonSelector
-}) {
-  const addButton = $(addButtonSelector);
+}: InputDuplicatorOptions) {
+  const addButton = $<HTMLButtonElement>(addButtonSelector);
   if (!addButton) {
     return;
   }
 
-  const form = addButton.closest('form');
-  const fieldRemover = (event, target) => {
+  const form = assertNotNull(addButton.closest('form'));
+  const fieldRemover = (event: MouseEvent, target: HTMLElement) => {
     event.preventDefault();
 
     // Prevent removing the final field element to not "brick" the form
@@ -33,7 +30,7 @@ function inputDuplicatorCreator({
       return;
     }
 
-    removeEl(target.closest(fieldSelector));
+    removeEl(assertNotNull(target.closest<HTMLElement>(fieldSelector)));
     enableEl(addButton);
   };
 
@@ -42,34 +39,40 @@ function inputDuplicatorCreator({
   });
 
 
-  const maxOptionCount = parseInt($(maxInputCountSelector, form).innerHTML, 10);
+  const maxOptionCountElement = assertNotNull($(maxInputCountSelector, form));
+  const maxOptionCount = parseInt(maxOptionCountElement.innerHTML, 10);
+
   addButton.addEventListener('click', e => {
     e.preventDefault();
 
-    const existingFields = $$(fieldSelector, form);
+    const existingFields = $$<HTMLElement>(fieldSelector, form);
     let existingFieldsLength = existingFields.length;
+
     if (existingFieldsLength < maxOptionCount) {
       // The last element matched by the `fieldSelector` will be the last field, make a copy
       const prevField = existingFields[existingFieldsLength - 1];
-      const prevFieldCopy = prevField.cloneNode(true);
-      const prevFieldCopyInputs = $$('input', prevFieldCopy);
-      prevFieldCopyInputs.forEach(prevFieldCopyInput => {
+      const prevFieldCopy = prevField.cloneNode(true) as HTMLElement;
+
+      $$<HTMLInputElement>('input', prevFieldCopy).forEach(prevFieldCopyInput => {
         // Reset new input's value
         prevFieldCopyInput.value = '';
         prevFieldCopyInput.removeAttribute('value');
+
         // Increment sequential attributes of the input
-        ['name', 'id'].forEach(attr => {
-          prevFieldCopyInput.setAttribute(attr, prevFieldCopyInput[attr].replace(/\d+/g, `${existingFieldsLength}`));
-        });
+        prevFieldCopyInput.setAttribute('name', prevFieldCopyInput.name.replace(/\d+/g, `${existingFieldsLength}`));
+        prevFieldCopyInput.setAttribute('id', prevFieldCopyInput.id.replace(/\d+/g, `${existingFieldsLength}`));
       });
 
+      const parentNode = assertNotNull(prevField.parentNode);
+      
       // Insert copy before the last field's next sibling, or if none, at the end of its parent
       if (prevField.nextElementSibling) {
-        prevField.parentNode.insertBefore(prevFieldCopy, prevField.nextElementSibling);
+        parentNode.insertBefore(prevFieldCopy, prevField.nextElementSibling);
       }
       else {
-        prevField.parentNode.appendChild(prevFieldCopy);
+        parentNode.appendChild(prevFieldCopy);
       }
+
       existingFieldsLength++;
     }
 
@@ -79,5 +82,3 @@ function inputDuplicatorCreator({
     }
   });
 }
-
-export { inputDuplicatorCreator };