Snapshot testing in React Native

Introduction

Snapshot testing helps you check if your app's screen or component looks the same as before. It catches unexpected changes easily.

When you want to make sure a UI component does not change after code updates.

When you add new features and want to confirm old screens still look right.

When fixing bugs to verify the fix does not break the UI.

When refactoring code to keep the UI consistent.

When collaborating with others to track UI changes clearly.

Syntax

React Native

import React from 'react';
import renderer from 'react-test-renderer';
import MyComponent from '../MyComponent';

test('renders correctly', () => {
  const tree = renderer.create(<MyComponent />).toJSON();
  expect(tree).toMatchSnapshot();
});

Use renderer.create() to render the component in a test environment.

toMatchSnapshot() compares the rendered output to a saved snapshot file.

Examples

Tests if the Button component renders the same as before.

React Native

test('button snapshot', () => {
  const tree = renderer.create(<Button title="Click" />).toJSON();
  expect(tree).toMatchSnapshot();
});

Checks if the TextInput component UI stays consistent.

React Native

test('text input snapshot', () => {
  const tree = renderer.create(<TextInput placeholder="Name" />).toJSON();
  expect(tree).toMatchSnapshot();
});

Sample App

This test renders a simple Greeting component and saves its UI structure as a snapshot. Later runs compare the UI to this snapshot to detect changes.

React Native

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

function Greeting() {
  return (
    <View>
      <Text>Hello, friend!</Text>
    </View>
  );
}

test('Greeting renders correctly', () => {
  const tree = renderer.create(<Greeting />).toJSON();
  expect(tree).toMatchSnapshot();
});

Important Notes

Snapshots are saved as files in a __snapshots__ folder next to your test files.

If UI changes are expected, update snapshots by running tests with --updateSnapshot flag.

Keep snapshots small and focused on one component to avoid large, hard-to-read files.

Summary

Snapshot testing helps catch unexpected UI changes quickly.

It saves a rendered UI output and compares future renders to it.

Use it to keep your app's look consistent during development.

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