react19-test-patterns - Claude MCP Skill

# React 19 Test Migration Patterns

Reference for all test file migrations required by React 19.

## Priority Order

Fix test files in this order; each layer depends on the previous:

1. **`act` import**  fix first, it unblocks everything else
2. **`Simulate` → `fireEvent`**  fix immediately after act
3. **Full react-dom/test-utils cleanup**  remove remaining imports
4. **StrictMode call counts**  measure actual, don't guess
5. **Async act wrapping**  for remaining "not wrapped in act" warnings
6. **Custom render helper**  verify once per codebase, not per test

---

## 1. act() Import Fix

```jsx
// Before  REMOVED in React 19:
import { act } from 'react-dom/test-utils';

// After:
import { act } from 'react';
```

If mixed with other test-utils imports:
```jsx
// Before:
import { act, Simulate, renderIntoDocument } from 'react-dom/test-utils';

// After  split the imports:
import { act } from 'react';
import { fireEvent, render } from '@testing-library/react'; // replaces Simulate + renderIntoDocument
```

---

## 2. Simulate → fireEvent

```jsx
// Before  Simulate REMOVED in React 19:
import { Simulate } from 'react-dom/test-utils';
Simulate.click(element);
Simulate.change(input, { target: { value: 'hello' } });
Simulate.submit(form);
Simulate.keyDown(element, { key: 'Enter', keyCode: 13 });

// After:
import { fireEvent } from '@testing-library/react';
fireEvent.click(element);
fireEvent.change(input, { target: { value: 'hello' } });
fireEvent.submit(form);
fireEvent.keyDown(element, { key: 'Enter', keyCode: 13 });
```

---

## 3. react-dom/test-utils Full API Map

| Old (react-dom/test-utils) | New location |
|---|---|
| `act` | `import { act } from 'react'` |
| `Simulate` | `fireEvent` from `@testing-library/react` |
| `renderIntoDocument` | `render` from `@testing-library/react` |
| `findRenderedDOMComponentWithTag` | `getByRole`, `getByTestId` from RTL |
| `findRenderedDOMComponentWithClass` | `getByRole` or `container.querySelector` |
| `scryRenderedDOMComponentsWithTag` | `getAllByRole` from RTL |
| `isElement`, `isCompositeComponent` | Remove  not needed with RTL |
| `isDOMComponent` | Remove |

---

## 4. StrictMode Call Count Fixes

React 19 StrictMode no longer double-invokes `useEffect` in development. Spy assertions counting effect calls must be updated.

**Strategy  always measure, never guess:**
```bash
# Run the failing test, read the actual count from the error:
npm test -- --watchAll=false --testPathPattern="[filename]" --forceExit 2>&1 | grep -E "Expected|Received"
```

```jsx
// Before (React 18 StrictMode  effects ran twice):
expect(mockFn).toHaveBeenCalledTimes(2);  // 1 call × 2 (strict double-invoke)

// After (React 19 StrictMode  effects run once):
expect(mockFn).toHaveBeenCalledTimes(1);
```

```jsx
// Render-phase calls (component body)  still double-invoked in React 19 StrictMode:
expect(renderSpy).toHaveBeenCalledTimes(2);  // stays at 2 for render body calls