test(ssr): add SSR and hydration tests for 'getServerSnapshot'

tests/ssr.test.tsx

@@ -0,0 +1,84 @@
+import { StrictMode } from 'react'
+import { act } from '@testing-library/react'
+import { hydrateRoot } from 'react-dom/client'
+import { renderToString } from 'react-dom/server'
+import { afterEach, describe, expect, it, vi } from 'vitest'
+import { proxy, useSnapshot } from 'valtio'
+
+describe('ssr', () => {
+  afterEach(() => {
+    document.body.innerHTML = ''
+  })
+
+  it('should return initial snapshot in SSR', () => {
+    const obj = proxy({ count: 0 })
+
+    const Counter = () => {
+      const snap = useSnapshot(obj)
+      return <div>count: {snap.count}</div>
+    }
+
+    const view = renderToString(
+      <StrictMode>
+        <Counter />
+      </StrictMode>,
+    )
+
+    expect(view).toContain('count:')
+    expect(view).toContain('0')
+  })
+
+  it('should return the latest snapshot after proxy state changes', () => {
+    const obj = proxy({ count: 0 })
+
+    obj.count = 5
+
+    const Counter = () => {
+      const snap = useSnapshot(obj)
+      return <div>count: {snap.count}</div>
+    }
+
+    const view = renderToString(
+      <StrictMode>
+        <Counter />
+      </StrictMode>,
+    )
+
+    expect(view).toContain('count:')
+    expect(view).toContain('5')
+  })
+
+  it('should not cause hydration mismatch', async () => {
+    const obj = proxy({ count: 0 })
+
+    const Counter = () => {
+      const snap = useSnapshot(obj)
+      return <div>count: {snap.count}</div>
+    }
+
+    const view = renderToString(
+      <StrictMode>
+        <Counter />
+      </StrictMode>,
+    )
+
+    const container = document.createElement('div')
+    container.innerHTML = view
+    document.body.appendChild(container)
+
+    const errorSpy = vi.spyOn(console, 'error')
+
+    await act(() =>
+      hydrateRoot(
+        container,
+        <StrictMode>
+          <Counter />
+        </StrictMode>,
+      ),
+    )
+
+    expect(errorSpy).not.toHaveBeenCalled()
+
+    errorSpy.mockRestore()
+  })
+})