Component testing in React Native

Introduction

Component testing helps check if parts of your app work right by themselves. It finds mistakes early so your app stays strong.

When you want to check if a button works before adding it to the app.

When you build a new screen and want to make sure it shows the right text.

When you fix a bug and want to confirm the fix works.

When you want to test how a component looks with different data.

When you want to avoid breaking other parts of the app after changes.

Syntax

React Native

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

test('description of what to test', () => {
  const { getByText } = render(<MyComponent />);
  expect(getByText('Hello')).toBeTruthy();
});

Use render to create the component in a test environment.

Use queries like getByText to find elements inside the component.

Examples

This test checks if the Button component shows the label passed as a prop.

React Native

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

test('button shows correct label', () => {
  const { getByText } = render(<Button label="Click me" />);
  expect(getByText('Click me')).toBeTruthy();
});

This test checks if the Greeting component displays the correct name.

React Native

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

test('greeting shows user name', () => {
  const { getByText } = render(<Greeting name="Anna" />);
  expect(getByText('Hello, Anna!')).toBeTruthy();
});

Sample App

This example shows a simple Welcome component that greets a user by name. The test checks if the greeting text appears correctly when the component is rendered with a user prop.

React Native

import React from 'react';
import { Text, View } from 'react-native';

export default function Welcome(props) {
  return (
    <View>
      <Text>Welcome, {props.user}!</Text>
    </View>
  );
}

// Test file
import React from 'react';
import { render } from '@testing-library/react-native';
import Welcome from './Welcome';

test('Welcome component shows user name', () => {
  const { getByText } = render(<Welcome user="Sam" />);
  expect(getByText('Welcome, Sam!')).toBeTruthy();
});

Important Notes

Always test one thing at a time in your component tests.

Use descriptive test names to know what each test checks.

Component tests run fast and help catch UI bugs early.

Summary

Component testing checks small parts of your app alone.

It uses tools like @testing-library/react-native to render and check components.

Good tests help keep your app working well as it grows.

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