react19-test-patterns
Provides before/after patterns for migrating test files to React 19 compatibility, including act() imports, Simulate removal, and StrictMode call count changes.
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- react19-test-patterns
- description
- Provides before/after patterns for migrating test files to React 19 compatibility, including act() imports, Simulate removal, and StrictMode call count changes.
# 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
```
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