testing-library

name	testing-library
description	Enforces best practices for unit testing with Jest, @testing-library/react-native, and jest-expo in Expo projects. This skill should be used when writing, reviewing, or debugging unit tests to ensure tests are accessible, maintainable, and follow Testing Library guiding principles. Use this skill for test file creation, query selection, async handling, mocking patterns, and Expo Router testing.

Testing Library Best Practices

Overview

This skill enforces best practices for unit testing in Expo applications using Jest, @testing-library/react-native, and jest-expo. Tests should be user-centric, accessible, and behavior-focused rather than implementation-focused.

Core Principles

1. Test User Behavior, Not Implementation

Focus on what the component does from a user's perspective, not how it achieves it internally.

// Correct - tests visible behavior
expect(screen.getByRole("button", { name: "Submit" })).toBeEnabled();

// Incorrect - tests implementation details
expect(component.state.isSubmitting).toBe(false);

2. Use Accessible Queries

Queries should reflect how users and assistive technologies interact with the UI.

// Correct - uses accessible role and name
screen.getByRole("button", { name: /save changes/i });

// Incorrect - relies on implementation detail
screen.getByTestId("save-btn");

3. One Assertion Per Behavior

Each test should verify one behavior. Multiple assertions are acceptable when verifying different aspects of the same behavior.

// Correct - focused test
test("displays error message when submission fails", async () => {
  render(<Form />);
  await userEvent.press(screen.getByRole("button", { name: "Submit" }));
  expect(await screen.findByRole("alert")).toHaveTextContent("Failed");
});

// Incorrect - testing multiple behaviors
test("form works correctly", async () => {
  // Tests validation, submission, success, and error handling...
});

4. Prefer userEvent Over fireEvent

userEvent simulates realistic user interactions including the full event sequence.

// Correct - realistic interaction
const user = userEvent.setup();
await user.press(screen.getByRole("button", { name: "Submit" }));

// Less ideal - simplified event
fireEvent.press(screen.getByRole("button", { name: "Submit" }));

Query Priority

Choose queries based on accessibility, following this priority order:

Priority	Query	When to Use
1	getByRole	Interactive elements, headings, buttons
2	getByLabelText	Form fields with labels
3	getByText	Non-interactive content, static text
4	getByTestId	Last resort when semantic queries fail

For detailed query patterns, see references/query-priority.md.

Async Testing Patterns

Use findBy for Async Assertions

// Correct - waits for element to appear
expect(await screen.findByRole("alert")).toBeOnTheScreen();

// Incorrect - may fail if element appears async
expect(screen.getByRole("alert")).toBeOnTheScreen();

Use waitFor for Side Effects

// Correct - assertion inside waitFor
await waitFor(() => {
  expect(mockCallback).toHaveBeenCalledWith("success");
});

// Incorrect - side effect inside waitFor
await waitFor(() => {
  fireEvent.press(button); // Never do this
});

For comprehensive async patterns, see references/async-patterns.md.

Mocking Patterns

Required Global Mocks

These must be configured in jest/setup-jest.ts:

// AsyncStorage
jest.mock("@react-native-async-storage/async-storage", () =>
  require("@react-native-async-storage/async-storage/jest/async-storage-mock")
);

// Expo Fonts (to avoid async icon assertions)
jest.mock("expo-font", () => ({
  ...jest.requireActual("expo-font"),
  isLoaded: jest.fn(() => true),
}));

For complete mocking patterns, see references/mocking-patterns.md.

Expo Router Testing

Use renderRouter from expo-router/testing-library instead of render when testing components that use Expo Router.

import { renderRouter, screen } from "expo-router/testing-library";

test("navigates to player detail", async () => {
  renderRouter({
    index: () => <PlayerList />,
    "players/[id]": () => <PlayerDetail />,
  });

  await userEvent.press(screen.getByRole("button", { name: "View Player" }));
  expect(screen).toHavePathname("/players/123");
});

For Expo Router testing details, see references/expo-router-testing.md.

Test Structure

File Organization

• Place test files in __tests__/ directories, not alongside source files
• Never place tests inside the app/ directory (Expo Router constraint)
• Use .test.ts or .test.tsx extensions

AAA Pattern

Structure every test with Arrange-Act-Assert:

test("increments counter when button pressed", async () => {
  // Arrange
  const user = userEvent.setup();
  render(<Counter initialCount={0} />);

  // Act
  await user.press(screen.getByRole("button", { name: "Increment" }));

  // Assert
  expect(screen.getByRole("text", { name: "Count: 1" })).toBeOnTheScreen();
});

Descriptive Test Names

Use descriptive names that explain the expected behavior:

// Correct - describes behavior
test("displays validation error when email format is invalid", () => {});
test("disables submit button while form is submitting", () => {});

// Incorrect - vague or implementation-focused
test("email validation works", () => {});
test("sets isSubmitting to true", () => {});

Jest Configuration

Manual React Native Resolution (No Preset)

Lisa configures Jest manually instead of using the jest-expo preset to avoid jsdom incompatibility with react-native/jest/setup.js. The configuration in jest.expo.ts provides haste, resolver, transform, and setupFiles that match the preset's behavior without redefining window.

Use Fake Timers with userEvent

jest.useFakeTimers();

test("handles debounced input", async () => {
  const user = userEvent.setup();
  render(<SearchInput />);

  await user.type(screen.getByRole("textbox"), "query");
  jest.runAllTimers();

  expect(await screen.findByText("Results")).toBeOnTheScreen();
});

Anti-Patterns

Never Test Implementation Details

// Wrong - testing internal state
expect(wrapper.state().isLoading).toBe(true);

// Wrong - testing component methods
expect(wrapper.instance().handleSubmit).toHaveBeenCalled();

// Correct - testing visible behavior
expect(screen.getByRole("progressbar")).toBeOnTheScreen();

Never Use getByTestId as Default

// Wrong - using testID when semantic query exists
screen.getByTestId("submit-button");

// Correct - using accessible query
screen.getByRole("button", { name: "Submit" });

Never Wrap render or fireEvent in act()

// Wrong - unnecessary act wrapper
await act(async () => {
  render(<Component />);
});

// Correct - render already wraps in act
render(<Component />);

Never Put Side Effects in waitFor

// Wrong - side effect in waitFor
await waitFor(() => {
  fireEvent.press(button);
  expect(result).toBeOnTheScreen();
});

// Correct - side effect before waitFor
fireEvent.press(button);
await waitFor(() => {
  expect(result).toBeOnTheScreen();
});

Never Use Multiple Assertions in waitFor

// Wrong - multiple assertions
await waitFor(() => {
  expect(title).toBeOnTheScreen();
  expect(subtitle).toBeOnTheScreen();
  expect(button).toBeEnabled();
});

// Correct - single assertion, chain with findBy
expect(await screen.findByRole("heading")).toBeOnTheScreen();
expect(screen.getByText("Subtitle")).toBeOnTheScreen();
expect(screen.getByRole("button")).toBeEnabled();

Quick Reference

Common Matchers

Matcher	Purpose
toBeOnTheScreen()	Element is currently rendered
toBeEnabled()	Interactive element is enabled
toBeDisabled()	Interactive element is disabled
toHaveTextContent()	Element contains text
toBeVisible()	Element is visible to user
toBeChecked()	Checkbox/radio is checked

Query Variants

Prefix	Returns	Throws on 0	Throws on >1	Async
getBy	Element	Yes	Yes	No
queryBy	Element | null	No	Yes	No
findBy	Promise<Element>	Yes	Yes	Yes
getAllBy	Element[]	Yes	No	No
queryAllBy	Element[]	No	No	No
findAllBy	Promise<Element[]>	Yes	No	Yes

References

• Query Priority - Detailed query selection guidance
• Async Patterns - Comprehensive async testing patterns
• Mocking Patterns - Common mocking configurations
• Expo Router Testing - Testing with Expo Router