React19 Test Patterns

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

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

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

If mixed with other test-utils imports:

// 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

// 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

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:

# Run the failing test, read the actual count from the error:
npm test -- --watchAll=false --testPathPattern="[filename]" --forceExit 2>&1 | grep -E "Expected|Received"

// 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);

// 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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**Source:** [`github/awesome-copilot`](https://github.com/github/awesome-copilot) → `skills/react19-test-patterns/SKILL.md`