diff --git a/assets/test/filter.test.ts b/assets/test/filter.test.ts
new file mode 100644
index 0000000..c116a3a
--- /dev/null
+++ b/assets/test/filter.test.ts
@@ -0,0 +1,152 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+import { init } from '../src/heatmap';
+import type { HeatmapData } from '../src/types';
+
+const MOCK_DATA: HeatmapData = {
+  days: [
+    { date: '2025-01-06', hours: 2.5, count: 3 },
+    { date: '2025-01-07', hours: 5.0, count: 5 },
+  ],
+  range: { begin: '2025-01-01', end: '2025-01-14' },
+};
+
+const EMPTY_DATA: HeatmapData = {
+  days: [],
+  range: { begin: '2025-01-01', end: '2025-01-14' },
+};
+
+const PROJECTS = [
+  { id: 1, name: 'Alpha' },
+  { id: 2, name: 'Beta' },
+];
+
+describe('filter dropdown', () => {
+  let container: HTMLDivElement;
+  let fetchMock: ReturnType<typeof vi.fn>;
+
+  beforeEach(() => {
+    container = document.createElement('div');
+    container.setAttribute('data-url', '/heatmap/data');
+    container.setAttribute('data-timesheet-url', '/en/timesheet/');
+    container.setAttribute('data-projects', JSON.stringify(PROJECTS));
+    document.body.appendChild(container);
+
+    fetchMock = vi.fn().mockResolvedValue({
+      ok: true,
+      json: () => Promise.resolve(MOCK_DATA),
+    });
+    vi.stubGlobal('fetch', fetchMock);
+  });
+
+  afterEach(() => {
+    document.body.removeChild(container);
+    vi.restoreAllMocks();
+  });
+
+  it('renders select with form-select class when projects exist', () => {
+    init(container);
+    const select = container.querySelector('select.form-select');
+    expect(select).not.toBeNull();
+  });
+
+  it('does not render select when no projects', () => {
+    container.setAttribute('data-projects', '[]');
+    init(container);
+    const select = container.querySelector('select.form-select');
+    expect(select).toBeNull();
+  });
+
+  it('has "All Projects" as first option', () => {
+    init(container);
+    const options = container.querySelectorAll('select.form-select option');
+    expect(options[0].textContent).toBe('All Projects');
+    expect((options[0] as HTMLOptionElement).value).toBe('');
+  });
+
+  it('has one option per project', () => {
+    init(container);
+    const options = container.querySelectorAll('select.form-select option');
+    expect(options.length).toBe(3); // All Projects + 2 projects
+    expect((options[1] as HTMLOptionElement).value).toBe('1');
+    expect(options[1].textContent).toBe('Alpha');
+    expect((options[2] as HTMLOptionElement).value).toBe('2');
+    expect(options[2].textContent).toBe('Beta');
+  });
+
+  it('has aria-label for accessibility', () => {
+    init(container);
+    const select = container.querySelector('select.form-select');
+    expect(select?.getAttribute('aria-label')).toBe('Filter by project');
+  });
+
+  it('fetches with project param on selection', async () => {
+    init(container);
+    await vi.waitFor(() => {
+      expect(fetchMock).toHaveBeenCalledTimes(1);
+    });
+
+    const select = container.querySelector('select.form-select') as HTMLSelectElement;
+    select.value = '1';
+    select.dispatchEvent(new Event('change'));
+
+    await vi.waitFor(() => {
+      expect(fetchMock).toHaveBeenCalledTimes(2);
+    });
+    expect(fetchMock.mock.calls[1][0]).toBe('/heatmap/data?project=1');
+  });
+
+  it('fetches without project param for All Projects', async () => {
+    init(container);
+    await vi.waitFor(() => expect(fetchMock).toHaveBeenCalledTimes(1));
+
+    const select = container.querySelector('select.form-select') as HTMLSelectElement;
+    select.value = '1';
+    select.dispatchEvent(new Event('change'));
+    await vi.waitFor(() => expect(fetchMock).toHaveBeenCalledTimes(2));
+
+    select.value = '';
+    select.dispatchEvent(new Event('change'));
+    await vi.waitFor(() => expect(fetchMock).toHaveBeenCalledTimes(3));
+    expect(fetchMock.mock.calls[2][0]).toBe('/heatmap/data');
+  });
+
+  it('re-renders heatmap after filter change', async () => {
+    init(container);
+    await vi.waitFor(() => {
+      expect(container.querySelector('rect.heatmap-cell')).not.toBeNull();
+    });
+
+    const select = container.querySelector('select.form-select') as HTMLSelectElement;
+    select.value = '1';
+    select.dispatchEvent(new Event('change'));
+
+    await vi.waitFor(() => {
+      expect(fetchMock).toHaveBeenCalledTimes(2);
+    });
+    await vi.waitFor(() => {
+      expect(container.querySelector('rect.heatmap-cell')).not.toBeNull();
+    });
+  });
+
+  it('shows filtered empty message when no data for project', async () => {
+    fetchMock
+      .mockResolvedValueOnce({ ok: true, json: () => Promise.resolve(MOCK_DATA) })
+      .mockResolvedValueOnce({ ok: true, json: () => Promise.resolve(EMPTY_DATA) });
+
+    init(container);
+    await vi.waitFor(() => {
+      expect(container.querySelector('rect.heatmap-cell')).not.toBeNull();
+    });
+
+    const select = container.querySelector('select.form-select') as HTMLSelectElement;
+    select.value = '1';
+    select.dispatchEvent(new Event('change'));
+
+    await vi.waitFor(() => {
+      expect(fetchMock).toHaveBeenCalledTimes(2);
+    });
+    await vi.waitFor(() => {
+      expect(container.textContent).toContain('No tracking data for this project');
+    });
+  });
+});