Add test for autofocus.js

One of the simplest of our JavaScript files to
test how difficult this is.

Answer is not too bad and includes the file
needing a DOM to operate on and jQuery in the
global scope.

tests/javascripts/autofocus.test.js

@@ -0,0 +1,84 @@
+beforeAll(() => {
+  // set up jQuery
+  window.jQuery = require('jquery');
+  $ = window.jQuery;
+
+  // load module code
+  require('govuk_frontend_toolkit/javascripts/govuk/modules.js');
+  require('../../app/assets/javascripts/autofocus.js');
+});
+
+afterAll(() => {
+  window.jQuery = null;
+  $ = null;
+
+  delete window.GOVUK;
+});
+
+describe('Autofocus', () => {
+
+  let focusHandler;
+  let search;
+
+  beforeEach(() => {
+
+    // set up DOM
+    document.body.innerHTML =
+      `<div data-module="autofocus">
+        <label class="form-label" for="search">
+          Search by name
+        </label>
+        <input autocomplete="off" class="form-control form-control-1-1" id="search" name="search" type="search" value="">
+      </div>`;
+
+    focusHandler = jest.fn();
+    search = document.getElementById('search');
+    search.addEventListener('focus', focusHandler, false);
+
+  });
+
+  afterEach(() => {
+
+    document.body.innerHTML = '';
+    search.removeEventListener('focus', focusHandler);
+    focusHandler = null;
+
+  });
+
+  test('is focused when modules start', () => {
+
+    // start module
+    window.GOVUK.modules.start();
+
+    expect(focusHandler).toHaveBeenCalled();
+
+  });
+
+  test('is not focused if the window has scrolled', () => {
+
+    // mock the window being scrolled 25px
+    $.prototype.scrollTop = jest.fn(() => 25);
+
+    // start module
+    window.GOVUK.modules.start();
+
+    expect(focusHandler).not.toHaveBeenCalled();
+
+  });
+
+  test('is focused if the window has scrolled but the force-focus flag is set', () => {
+
+    // mock the window being scrolled 25px
+    $.prototype.scrollTop = jest.fn(() => 25);
+
+    // set the force-focus flag
+    document.querySelector('div').setAttribute('data-force-focus', true);
+
+    // start module
+    window.GOVUK.modules.start();
+
+    expect(focusHandler).toHaveBeenCalled();
+
+  });
+
+});