diff --git a/tests/javascripts/main.test.js b/tests/javascripts/main.test.js
index e69de29bb..973a51d5c 100644
--- a/tests/javascripts/main.test.js
+++ b/tests/javascripts/main.test.js
@@ -0,0 +1,108 @@
+/**
+ * @jest-environment jsdom
+ */
+
+describe('Main init script', () => {
+    beforeEach(() => {
+        // Clear DOM
+        document.body.innerHTML = '';
+        jest.resetModules();
+        jest.clearAllMocks();
+    });
+
+    it('calls GOVUK.Frontend.initAll if available', () => {
+        const initAllMock = jest.fn();
+        const showHideInitMock = jest.fn();
+
+        global.GOVUK = {
+            Frontend: { initAll: initAllMock },
+            ShowHideContent: jest.fn().mockImplementation(() => ({
+            init: showHideInitMock
+            }))
+        };
+
+        require('../../app/assets/javascripts/main.js');
+
+        expect(initAllMock).toHaveBeenCalled();
+    });
+
+    it('initializes GOVUK.ShowHideContent', () => {
+        const initMock = jest.fn();
+        const ShowHideContentMock = jest.fn().mockImplementation(() => ({
+            init: initMock
+        }));
+        global.GOVUK = {
+            ShowHideContent: ShowHideContentMock
+        };
+
+        require('../../app/assets/javascripts/main.js');
+
+        expect(ShowHideContentMock).toHaveBeenCalled();
+        expect(initMock).toHaveBeenCalled();
+    });
+
+    it('focuses input next to error message inside label', () => {
+        document.body.innerHTML = `
+            <form>
+            <label>
+                <span class="error-message">Required</span>
+            </label>
+            <input id="name" />
+            </form>
+        `;
+        const focusMock = jest.fn();
+        const input = document.querySelector('input');
+        input.focus = focusMock;
+
+        require('../../app/assets/javascripts/main.js');
+
+        // simulate DOMContentLoaded
+        document.dispatchEvent(new Event('DOMContentLoaded'));
+
+        expect(focusMock).toHaveBeenCalled();
+    });
+
+    it('adds border color on header click', () => {
+        document.body.innerHTML = `
+          <div class="govuk-header__container"></div>
+        `;
+
+        global.GOVUK = {
+          ShowHideContent: jest.fn().mockImplementation(() => ({ init: jest.fn() })),
+          Frontend: { initAll: jest.fn() }
+        };
+
+        require('../../app/assets/javascripts/main.js');
+        document.dispatchEvent(new Event('DOMContentLoaded'));
+
+        const header = document.querySelector('.govuk-header__container');
+        header.click();
+
+        expect(header.style.borderColor).toBe('#005ea5');
+    });
+
+    it('adds and removes class on parent during focusin and focusout', () => {
+        document.body.innerHTML = `
+        <div class="js-mark-focus-on-parent">
+          <input type="text" />
+        </div>
+      `;
+
+        global.GOVUK = {
+            ShowHideContent: jest.fn().mockImplementation(() => ({ init: jest.fn() })),
+            Frontend: { initAll: jest.fn() }
+        };
+
+        require('../../app/assets/javascripts/main.js');
+        document.dispatchEvent(new Event('DOMContentLoaded'));
+
+        const container = document.querySelector('.js-mark-focus-on-parent');
+        const input = container.querySelector('input');
+
+        input.dispatchEvent(new FocusEvent('focusin', { bubbles: true }));
+        expect(container.classList.contains('js-child-has-focus')).toBe(true);
+
+        input.dispatchEvent(new FocusEvent('focusout', { bubbles: true }));
+        expect(container.classList.contains('js-child-has-focus')).toBe(false);
+    });
+});