Component testing in React Native - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is component testing in React Native?

Component testing checks if a single UI component works correctly by itself, without involving the whole app.

intermediate

Why do we use shallow rendering in component testing?

Shallow rendering tests a component without rendering its child components, making tests faster and focused.

beginner

Which library is commonly used for React Native component testing?

React Native Testing Library is popular because it encourages testing components like users interact with them.

intermediate

What does a snapshot test do in component testing?

A snapshot test saves the rendered output of a component and compares it later to detect unexpected changes.

intermediate

How can you simulate a button press in React Native component tests?

You can simulate a button press by firing a press event using fireEvent.press from React Native Testing Library.

What is the main goal of component testing in React Native?

A Test backend API responses

B Test the entire app navigation

C Test device hardware features

D Test a single UI component in isolation

Component testing focuses on checking one UI component alone to ensure it works as expected.

Which method helps to avoid rendering child components during tests?

A Shallow rendering

B Full rendering

C Snapshot testing

D Integration testing

Shallow rendering renders only the component itself, not its children, making tests simpler.

Which library is best suited for user-focused React Native component testing?

A Mocha

B Jest DOM

C React Native Testing Library

D Enzyme

React Native Testing Library encourages testing components as users interact with them.

What does a snapshot test compare?

A Rendered output over time

B Component code style

C API response times

D Device compatibility

Snapshot tests compare the saved rendered output to the current output to catch changes.

How do you simulate a button press in React Native Testing Library?

A triggerEvent(buttonElement, 'click')

B fireEvent.press(buttonElement)

C simulatePress(buttonElement)

D buttonElement.click()

fireEvent.press simulates a press event on a button in React Native Testing Library.

Explain the purpose and benefits of component testing in React Native.

Think about testing one piece of the UI alone before the whole app.

Describe how you would write a simple test to check if a button press calls a function in React Native.

Use React Native Testing Library and fireEvent.press.

Practice

1. What is the main purpose of component testing in React Native?

easy

A. To check the app's network connectivity

B. To test the entire app's performance under load

C. To verify the app's installation process

D. To check small parts of the app work correctly on their own

Solution

1. Step 1: Understand component testing scope

   Component testing focuses on small parts, or components, of the app individually.

2. Step 2: Compare options to purpose

   Only To check small parts of the app work correctly on their own describes testing small parts alone, which matches component testing.

3. Final Answer:

   To check small parts of the app work correctly on their own -> Option D

4. Quick Check:

   Component testing = small parts tested alone [OK]

Hint: Component testing checks parts, not whole app [OK]

Common Mistakes:

• Confusing component testing with full app testing
• Thinking it tests app installation or network
• Assuming it tests performance or load

2. Which of the following is the correct way to import the render function from @testing-library/react-native?

easy

A. import { render } from '@testing-library/react-native';

B. import render from '@testing-library/react-native';

C. import * as render from '@testing-library/react-native';

D. import { Render } from '@testing-library/react-native';

Solution

1. Step 1: Recall correct import syntax for named exports

   Named exports are imported with curly braces, like { render }.

2. Step 2: Check each option

   import { render } from '@testing-library/react-native'; uses correct named import syntax; others are incorrect or use wrong casing.

3. Final Answer:

   import { render } from '@testing-library/react-native'; -> Option A

4. Quick Check:

   Named import uses curly braces [OK]

Hint: Use curly braces for named imports like render [OK]

Common Mistakes:

• Using default import syntax for named exports
• Wrong casing in import names
• Using * as import incorrectly

3. Given this test code, what will be the output of the test?

import React from 'react';
import { render } from '@testing-library/react-native';
import { Text } from 'react-native';

function Greeting() {
  return Hello, World!;
}

const { getByText } = render();
const element = getByText('Hello, World!');
console.log(element.props.children);

medium

A. undefined

B. Hello, World!

C. Error: Text not found

D. null

Solution

1. Step 1: Understand render and getByText usage

   The render function creates the component tree; getByText finds the Text with given string.

2. Step 2: Check what element.props.children contains

   The Text component's children is the string 'Hello, World!', so element.props.children is that string.

3. Final Answer:

   Hello, World! -> Option B

4. Quick Check:

   getByText finds text content correctly [OK]

Hint: getByText returns element with matching text [OK]

Common Mistakes:

• Assuming getByText returns undefined if found
• Confusing element props with state
• Expecting error when text exists

4. What is wrong with this test code snippet?

import { render } from '@testing-library/react-native';

const { getByText } = render();
const button = getByText('Submit');
button.onPress();

medium

A. You cannot call onPress() directly on the element returned by getByText

B. render should be imported from 'react-native' not '@testing-library/react-native'

C. getByText should be called with a component, not a string

D. MyComponent must be wrapped in a <View> to render

Solution

1. Step 1: Understand what getByText returns

   getByText returns a React Test Instance, not a real component with event handlers.

2. Step 2: Check how to simulate press events

   To simulate press, use fireEvent.press(button) from testing library, not call onPress() directly.

3. Final Answer:

   You cannot call onPress() directly on the element returned by getByText -> Option A

4. Quick Check:

   Use fireEvent.press, not direct onPress call [OK]

Hint: Use fireEvent.press to simulate button press [OK]

Common Mistakes:

• Calling onPress() directly on test element
• Importing render from wrong package
• Misusing getByText argument type

5. You want to test a component that conditionally renders a <Text> with 'Logged In' only if the prop isLoggedIn is true. Which test code correctly checks this behavior?

hard

A. const { getByText } = render();
expect(getByText('Logged In')).toBeNull();

B. const { getByText } = render();
expect(queryByText('Logged In')).not.toBeNull();

C. const { queryByText } = render();
expect(queryByText('Logged In')).toBeNull();

D. const { queryByText } = render();
expect(getByText('Logged In')).toBeNull();

Solution

1. Step 1: Understand difference between getByText and queryByText

   getByText throws error if text not found; queryByText returns null if not found.

2. Step 2: Check test logic for isLoggedIn false

   When isLoggedIn is false, 'Logged In' text should not appear, so queryByText returns null.

3. Final Answer:

   const { queryByText } = render(<MyComponent isLoggedIn={false} />); expect(queryByText('Logged In')).toBeNull(); -> Option C

4. Quick Check:

   Use queryByText to check absence safely [OK]

Hint: Use queryByText to check if element is missing [OK]

Common Mistakes:

• Using getByText when element may be missing (causes error)
• Mixing getByText and queryByText incorrectly
• Expecting getByText to return null