diff --git a/assets/js/__tests__/input-duplicator.spec.ts b/assets/js/__tests__/input-duplicator.spec.ts
new file mode 100644
index 000000000..fc7adf0b7
--- /dev/null
+++ b/assets/js/__tests__/input-duplicator.spec.ts
@@ -0,0 +1,91 @@
+import { inputDuplicatorCreator } from '../input-duplicator';
+import { assertNotNull } from '../utils/assert';
+import { $, $$, removeEl } from '../utils/dom';
+
+describe('Input duplicator functionality', () => {
+  beforeEach(() => {
+    document.documentElement.insertAdjacentHTML('beforeend', `<form action="/">
+      <div class="js-max-input-count">3</div>
+      <div class="js-input-source">
+        <input id="0" name="0" class="js-input" type="text"/>
+        <label>
+          <a href="#" class="js-remove-input">Delete</a>
+        </label>
+      </div>
+      <div class="js-button-container">
+        <button type="button" class="js-add-input">Add input</button>
+      </div>
+    </form>`);
+  });
+
+  afterEach(() => {
+    removeEl($$<HTMLFormElement>('form'));
+  });
+
+  function runCreator() {
+    inputDuplicatorCreator({
+      addButtonSelector: '.js-add-input',
+      fieldSelector: '.js-input-source',
+      maxInputCountSelector: '.js-max-input-count',
+      removeButtonSelector: '.js-remove-input',
+    });
+  }
+
+  it('should ignore forms without a duplicator button', () => {
+    removeEl($$<HTMLButtonElement>('button'));
+    expect(runCreator()).toBeUndefined();
+  });
+
+  it('should duplicate the input elements', () => {
+    runCreator();
+
+    expect($$('input')).toHaveLength(1);
+
+    assertNotNull($<HTMLButtonElement>('.js-add-input')).click();
+
+    expect($$('input')).toHaveLength(2);
+  });
+
+  it('should duplicate the input elements when the button is before the inputs', () => {
+    const form = assertNotNull($<HTMLFormElement>('form'));
+    const buttonDiv = assertNotNull($<HTMLDivElement>('.js-button-container'));
+    removeEl(buttonDiv);
+    form.insertAdjacentElement('afterbegin', buttonDiv);
+    runCreator();
+
+    assertNotNull($<HTMLButtonElement>('.js-add-input')).click();
+
+    expect($$('input')).toHaveLength(2);
+  });
+
+  it('should not create more input elements than the limit', () => {
+    runCreator();
+
+    for (let i = 0; i < 5; i += 1) {
+      assertNotNull($<HTMLButtonElement>('.js-add-input')).click();
+    }
+
+    expect($$('input')).toHaveLength(3);
+  });
+
+  it('should remove duplicated input elements', () => {
+    runCreator();
+
+    assertNotNull($<HTMLButtonElement>('.js-add-input')).click();
+    assertNotNull($<HTMLAnchorElement>('.js-remove-input')).click();
+
+    expect($$('input')).toHaveLength(1);
+  });
+
+  it('should not remove the last input element', () => {
+    runCreator();
+
+    assertNotNull($<HTMLAnchorElement>('.js-remove-input')).click();
+    assertNotNull($<HTMLAnchorElement>('.js-remove-input')).click();
+    for (let i = 0; i < 5; i += 1) {
+      assertNotNull($<HTMLAnchorElement>('.js-remove-input')).click();
+    }
+
+    expect($$('input')).toHaveLength(1);
+  });
+});
diff --git a/assets/js/input-duplicator.js b/assets/js/input-duplicator.js
deleted file mode 100644
index 2ffa89bc1..000000000
--- a/assets/js/input-duplicator.js
+++ /dev/null
@@ -1,83 +0,0 @@
-import { $, $$, 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({
-  addButtonSelector,
-  fieldSelector,
-  maxInputCountSelector,
-  removeButtonSelector
-}) {
-  const addButton = $(addButtonSelector);
-  if (!addButton) {
-    return;
-  }
-
-  const form = addButton.closest('form');
-  const fieldRemover = (event, target) => {
-    event.preventDefault();
-
-    // Prevent removing the final field element to not "brick" the form
-    const existingFields = $$(fieldSelector, form);
-    if (existingFields.length <= 1) {
-      return;
-    }
-
-    removeEl(target.closest(fieldSelector));
-    enableEl(addButton);
-  };
-
-  delegate(document, 'click', {
-    [removeButtonSelector]: leftClick(fieldRemover)
-  });
-
-
-  const maxOptionCount = parseInt($(maxInputCountSelector, form).innerHTML, 10);
-  addButton.addEventListener('click', e => {
-    e.preventDefault();
-
-    const existingFields = $$(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 = $$('input', prevFieldCopy);
-      prevFieldCopyInputs.forEach(prevFieldCopyInput => {
-        // Reset new input's value
-        prevFieldCopyInput.value = '';
-        prevFieldCopyInput.removeAttribute('value');
-        // Increment sequential attributes of the input
-        ['name', 'id'].forEach(attr => {
-          prevFieldCopyInput.setAttribute(attr, prevFieldCopyInput[attr].replace(/\d+/g, `${existingFieldsLength}`));
-        });
-      });
-
-      // Insert copy before the last field's next sibling, or if none, at the end of its parent
-      if (prevField.nextElementSibling) {
-        prevField.parentNode.insertBefore(prevFieldCopy, prevField.nextElementSibling);
-      }
-      else {
-        prevField.parentNode.appendChild(prevFieldCopy);
-      }
-      existingFieldsLength++;
-    }
-
-    // Remove the button if we reached the max number of options
-    if (existingFieldsLength >= maxOptionCount) {
-      disableEl(addButton);
-    }
-  });
-}
-
-export { inputDuplicatorCreator };
diff --git a/assets/js/input-duplicator.ts b/assets/js/input-duplicator.ts
new file mode 100644
index 000000000..e82c892d7
--- /dev/null
+++ b/assets/js/input-duplicator.ts
@@ -0,0 +1,76 @@
+import { assertNotNull } from './utils/assert';
+import { $, $$, disableEl, enableEl, removeEl } from './utils/dom';
+import { delegate, leftClick } from './utils/events';
+
+export interface InputDuplicatorOptions {
+  addButtonSelector: string;
+  fieldSelector: string;
+  maxInputCountSelector: string;
+  removeButtonSelector: string;
+}
+
+export function inputDuplicatorCreator({
+  addButtonSelector,
+  fieldSelector,
+  maxInputCountSelector,
+  removeButtonSelector
+}: InputDuplicatorOptions) {
+  const addButton = $<HTMLButtonElement>(addButtonSelector);
+  if (!addButton) {
+    return;
+  }
+
+  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
+    const existingFields = $$(fieldSelector, form);
+    if (existingFields.length <= 1) {
+      return;
+    }
+
+    removeEl(assertNotNull(target.closest<HTMLElement>(fieldSelector)));
+    enableEl(addButton);
+  };
+
+  delegate(form, 'click', {
+    [removeButtonSelector]: leftClick(fieldRemover)
+  });
+
+
+  const maxOptionCountElement = assertNotNull($(maxInputCountSelector, form));
+  const maxOptionCount = parseInt(maxOptionCountElement.innerHTML, 10);
+
+  addButton.addEventListener('click', e => {
+    e.preventDefault();
+
+    const existingFields = $$<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) as HTMLElement;
+
+      $$<HTMLInputElement>('input', prevFieldCopy).forEach(prevFieldCopyInput => {
+        // Reset new input's value
+        prevFieldCopyInput.value = '';
+        prevFieldCopyInput.removeAttribute('value');
+
+        // Increment sequential attributes of the input
+        prevFieldCopyInput.setAttribute('name', prevFieldCopyInput.name.replace(/\d+/g, `${existingFieldsLength}`));
+        prevFieldCopyInput.setAttribute('id', prevFieldCopyInput.id.replace(/\d+/g, `${existingFieldsLength}`));
+      });
+
+      prevField.insertAdjacentElement('afterend', prevFieldCopy);
+
+      existingFieldsLength++;
+    }
+
+    // Remove the button if we reached the max number of options
+    if (existingFieldsLength >= maxOptionCount) {
+      disableEl(addButton);
+    }
+  });
+}