Add a unit test for the useDebounce hook timing and cleanup

src/hooks/__tests__/useDebounce.test.ts

@@ -0,0 +1,116 @@
+import { act, renderHook } from '@testing-library/react';
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+
+import { useDebounce } from '../useDebounce';
+
+describe('useDebounce', () => {
+  beforeEach(() => {
+    vi.useFakeTimers();
+  });
+
+  afterEach(() => {
+    vi.useRealTimers();
+  });
+
+  it('returns the initial value immediately before the delay elapses', () => {
+    const { result } = renderHook(() => useDebounce('hello', 300));
+
+    expect(result.current).toBe('hello');
+  });
+
+  it('updates the debounced value only after the specified delay', () => {
+    const { result, rerender } = renderHook(
+      ({ value, delay }: { value: string; delay: number }) =>
+        useDebounce(value, delay),
+      { initialProps: { value: 'initial', delay: 300 } },
+    );
+
+    expect(result.current).toBe('initial');
+
+    rerender({ value: 'updated', delay: 300 });
+
+    // Before the delay elapses the debounced value is still stale.
+    expect(result.current).toBe('initial');
+
+    act(() => {
+      vi.advanceTimersByTime(300);
+    });
+
+    expect(result.current).toBe('updated');
+  });
+
+  it('coalesces rapid successive updates to the final value', () => {
+    const { result, rerender } = renderHook(
+      ({ value }: { value: string }) => useDebounce(value, 300),
+      { initialProps: { value: 'first' } },
+    );
+
+    rerender({ value: 'second' });
+    rerender({ value: 'third' });
+    rerender({ value: 'final' });
+
+    // No timer has fired yet — value should remain 'first'.
+    expect(result.current).toBe('first');
+
+    act(() => {
+      vi.advanceTimersByTime(300);
+    });
+
+    // Only the last value should be reflected.
+    expect(result.current).toBe('final');
+  });
+
+  it('does not update the debounced value after unmount (cleanup clears the timer)', () => {
+    const { result, unmount, rerender } = renderHook(
+      ({ value }: { value: string }) => useDebounce(value, 300),
+      { initialProps: { value: 'before' } },
+    );
+
+    rerender({ value: 'after' });
+
+    // Unmount before the timer fires.
+    unmount();
+
+    act(() => {
+      vi.advanceTimersByTime(300);
+    });
+
+    // The hook has been unmounted; the last captured value should still be
+    // 'before' because the pending timer was cleared on cleanup.
+    expect(result.current).toBe('before');
+  });
+
+  it('uses a default delay of 300 ms when none is provided', () => {
+    const { result, rerender } = renderHook(
+      ({ value }: { value: number }) => useDebounce(value),
+      { initialProps: { value: 0 } },
+    );
+
+    rerender({ value: 42 });
+
+    act(() => {
+      vi.advanceTimersByTime(299);
+    });
+    expect(result.current).toBe(0);
+
+    act(() => {
+      vi.advanceTimersByTime(1);
+    });
+    expect(result.current).toBe(42);
+  });
+
+  it('handles a delay of 0 and resolves synchronously within the same tick', () => {
+    const { result, rerender } = renderHook(
+      ({ value }: { value: string }) => useDebounce(value, 0),
+      { initialProps: { value: 'start' } },
+    );
+
+    rerender({ value: 'end' });
+
+    act(() => {
+      vi.advanceTimersByTime(0);
+    });
+
+    expect(result.current).toBe('end');
+  });
+});