Snapshot testing in React Native - Mini App: Build & Ship

Build: Simple Button Screen

A screen with a button that changes text when pressed. We will create a snapshot test to verify the UI does not change unexpectedly.

Target UI

┌───────────────────────────┐
│       Simple Button       │
│                           │
│       [ Press me ]         │
│                           │
│ Text: Not pressed yet     │
└───────────────────────────┘

Display a button labeled 'Press me'.

Show a text below the button that says 'Not pressed yet' initially.

When the button is pressed, change the text to 'Button pressed!'.

Write a snapshot test to capture the initial UI and verify it matches the expected snapshot.

Starter Code

React Native

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

export default function SimpleButtonScreen() {
  const [pressed, setPressed] = useState(false);

  return (
    <View style={styles.container}>
      {/* TODO: Add Button here */}
      {/* TODO: Add Text here to show pressed state */}
    </View>
  );
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    justifyContent: 'center',
    alignItems: 'center',
    padding: 16
  }
});

Task 1

Task 2

Solution

React Native

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

export default function SimpleButtonScreen() {
  const [pressed, setPressed] = useState(false);

  return (
    <View style={styles.container}>
      <Button title="Press me" onPress={() => setPressed(true)} />
      <Text style={styles.text} accessibilityRole="text">
        {pressed ? 'Button pressed!' : 'Not pressed yet'}
      </Text>
    </View>
  );
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    justifyContent: 'center',
    alignItems: 'center',
    padding: 16
  },
  text: {
    marginTop: 20,
    fontSize: 18
  }
});

This screen uses React Native's useState hook to track if the button was pressed.

The Button component triggers setPressed(true) on press, changing the text below it.

The Text component updates to show the current state.

For snapshot testing, you capture the initial render (before pressing) to ensure the UI stays consistent over time.

Final Result

Completed Screen

┌───────────────────────────┐
│       Simple Button       │
│                           │
│       [ Press me ]         │
│                           │
│ Text: Not pressed yet     │
└───────────────────────────┘

When user taps 'Press me', the text changes to 'Button pressed!'.

Snapshot test captures the initial UI with 'Not pressed yet' text.

Stretch Goal

Write a snapshot test using Jest and React Test Renderer to verify the initial UI of SimpleButtonScreen.

💡 Hint

Use 'react-test-renderer' to create a snapshot of the component and expect it to match the saved snapshot file.

Practice

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

easy

A. To check the app's performance speed

B. To test network requests and API responses

C. To detect unexpected changes in the UI output

D. To validate user input formats

Solution

1. Step 1: Understand snapshot testing goal

   Snapshot testing saves the UI output to detect changes later.

2. Step 2: Identify what snapshot testing checks

   It compares future UI renders to saved snapshots to catch unexpected changes.

3. Final Answer:

   To detect unexpected changes in the UI output -> Option C

4. Quick Check:

   Snapshot testing = UI change detection [OK]

Hint: Snapshot testing checks UI changes, not logic or speed [OK]

Common Mistakes:

• Confusing snapshot testing with API or performance tests
• Thinking it tests user input validation
• Assuming it measures app speed

2. Which of the following is the correct way to write a basic snapshot test in React Native using Jest?

easy

A. render().toMatchSnapshot();

B. expect(render()).toMatchSnapshot();

C. expect().toMatchSnapshot();

D. expect(render()).toBeSnapshot();

Solution

1. Step 1: Recall Jest snapshot syntax

   Jest requires wrapping the rendered output in expect() and calling toMatchSnapshot().

2. Step 2: Check each option's syntax

   expect(render()).toMatchSnapshot(); correctly wraps render output in expect() and calls toMatchSnapshot(). Others miss expect() or use wrong matcher.

3. Final Answer:

   expect(render()).toMatchSnapshot(); -> Option B

4. Quick Check:

   Use expect(render()).toMatchSnapshot() [OK]

Hint: Always wrap render output in expect() before toMatchSnapshot() [OK]

Common Mistakes:

• Omitting expect() around render output
• Using incorrect matcher like toBeSnapshot()
• Passing component directly to expect() without render

3. Given this snapshot test code:

const tree = render().toJSON();
expect(tree).toMatchSnapshot();

What will happen if the Button's title changes to "Press" in the next test run?

medium

A. The test will fail because the snapshot does not match

B. The test will pass because title changes are ignored

C. The test will throw a syntax error

D. The test will pass but log a warning

Solution

1. Step 1: Understand snapshot comparison

   Snapshots save the UI output including props like title. Changing title changes output.

2. Step 2: Effect of changing title on test

   Since the new render differs from saved snapshot, Jest will fail the test to alert the change.

3. Final Answer:

   The test will fail because the snapshot does not match -> Option A

4. Quick Check:

   Changed UI props cause snapshot test failure [OK]

Hint: Any UI change breaks snapshot test unless updated [OK]

Common Mistakes:

• Thinking snapshot ignores prop changes
• Expecting test to pass with changed UI
• Confusing snapshot failure with syntax errors

4. You wrote this snapshot test:

test('renders correctly', () => {
  const tree = render();
  expect(tree).toMatchSnapshot();
});

But Jest throws an error: expect(received).toMatchSnapshot is not a function. What is the likely cause?

medium

A. You should use toBeSnapshot() instead

B. You need to import toMatchSnapshot manually

C. The component MyComponent is missing props

D. You forgot to call .toJSON() on the render output

Solution

1. Step 1: Understand render output type

   render() returns an object, not a JSON serializable tree. toMatchSnapshot() works on JSON.

2. Step 2: Fix by calling toJSON()

   Calling toJSON() converts render output to snapshot-friendly format, enabling toMatchSnapshot() usage.

3. Final Answer:

   You forgot to call .toJSON() on the render output -> Option D

4. Quick Check:

   Call render().toJSON() before expect() [OK]

Hint: Always call .toJSON() on render output before snapshot [OK]

Common Mistakes:

• Skipping .toJSON() call on render output
• Trying to import toMatchSnapshot manually
• Using wrong matcher like toBeSnapshot()

5. You want to update your snapshot after intentionally changing your component's UI. Which command should you run to update the snapshot files?

hard

A. jest -u

B. jest --updateSnapshots

C. jest --updateSnapshot

D. jest --update-snapshots

Solution

1. Step 1: Recall Jest snapshot update command

   The correct Jest flag to update snapshots is -u or --updateSnapshot.

2. Step 2: Identify exact command

   Among options, only jest -u is the correct short form. Others are invalid or misspelled.

3. Final Answer:

   jest -u -> Option A

4. Quick Check:

   Use jest -u to update snapshots [OK]

Hint: Use jest -u to update snapshots after UI changes [OK]

Common Mistakes:

• Typing incorrect or misspelled flags
• Trying to update snapshots without flags
• Confusing updateSnapshot with updateSnapshots