React19 Test Patterns

React 19 test migration patterns

Convert React test files to React 19-compatible APIs in dependency order, preserving test intent while replacing removed react-dom/test-utils patterns and measuring StrictMode behavior instead of guessing call counts.

When to invoke

• "Migrate these tests for React 19."
• "Replace react-dom/test-utils imports in my test files."
• "Convert Simulate calls to Testing Library fireEvent."
• "Fix React 19 StrictMode call count failures."
• "Resolve not wrapped in act warnings after the React 19 upgrade."

Procedure

Fix test files in this order because each layer unblocks the next:

1. Fix act imports first.
2. Replace Simulate with fireEvent.
3. Remove all remaining react-dom/test-utils imports.
4. Measure and update StrictMode call counts.
5. Add async act or await only for remaining warnings.
6. Verify the custom render helper once per codebase, not separately in every test.

API migration map

before/after patterns

act() import

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

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

When mixed with other test-utils imports, split the imports and replace the removed helpers:

// Before
import { act, Simulate, renderIntoDocument } from 'react-dom/test-utils';

// After
import { act } from 'react';
import { fireEvent, render } from '@testing-harness/github-copilot/react';

Simulate to fireEvent

// Before: Simulate is 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-harness/github-copilot/react';
fireEvent.click(element);
fireEvent.change(input, { target: { value: 'hello' } });
fireEvent.submit(form);
fireEvent.keyDown(element, { key: 'Enter', keyCode: 13 });

StrictMode call counts

React 19 StrictMode no longer double-invokes useEffect in development, but render-phase calls such as component body spies can still be double-invoked. Measure actual failures.

npm test -- --watchAll=false --testPathPattern="[filename]" --forceExit 2>&1 | grep -E "Expected|Received"

// Before: React 18 StrictMode effects ran twice
expect(mockFn).toHaveBeenCalledTimes(2);

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

// Render-phase calls can still be double-invoked
expect(renderSpy).toHaveBeenCalledTimes(2);

Gotchas

• Measure StrictMode counts: do not globally replace 2 with 1; effect calls and render-phase calls differ.

• Do not leave partial test-utils imports: any remaining react-dom/test-utils import can hide removed APIs.

• Prefer user-event for user flows: use fireEvent for direct mechanical replacement, but preserve existing higher-level userEvent patterns when present.

• Async warnings often need awaited user actions: after upgrading @testing-harness/github-copilot/user-event, many calls require await before assertions.

• StrictMode wording matters: React 18 double-invokes effects; React 19 changes effect behavior but can still double-invoke render-phase work.

Output template

## React 19 test migration result

**Status:** migrated | changes required | blocked
**Files reviewed:** <test files>
**Primary pattern:** act import | Simulate removal | test-utils cleanup | StrictMode counts | async act

| File | Old API or failure | Change made | Validation |
| --- | --- | --- | --- |
| `<test file>` | `<old pattern>` | `<new pattern>` | `<test command/result>` |

Quality gate

• act imports come from react, not react-dom/test-utils.
• Simulate calls are replaced with fireEvent or an existing userEvent pattern.
• No remaining react-dom/test-utils imports exist in migrated files.
• StrictMode call counts are based on observed test output, not guessing.
• Async user interactions and act warnings are awaited or wrapped correctly.
• The output follows ## Output template exactly.