Snapshot testing in React Native - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is snapshot testing in React Native?

Snapshot testing captures the rendered output of a component and saves it as a file. Later, it compares the current output to the saved snapshot to detect unexpected changes.

beginner

How do you create a snapshot test for a React Native component?

You render the component using a test renderer, then use expect(tree).toMatchSnapshot() to save or compare the snapshot.

beginner

Why is snapshot testing useful in mobile app development?

It helps catch UI changes early by comparing the current UI output to a saved version, preventing accidental visual bugs.

intermediate

What should you do if a snapshot test fails because of an intentional UI change?

Review the changes and if they are expected, update the snapshot file by running the test command with the update flag (e.g., jest -u).

intermediate

Name one limitation of snapshot testing.

Snapshot tests can pass even if the UI looks wrong, because they only compare output text, not actual rendered visuals or user experience.

What does a snapshot test compare in React Native?

A The rendered component output to a saved snapshot file

B The app's performance metrics

C The network requests made by the app

D The app's database contents

Snapshot tests compare the current rendered output of a component to a previously saved snapshot file to detect changes.

Which command updates snapshot files when UI changes are intentional?

A jest --watch

B jest -u

C npm start

D react-native run-ios

The jest -u command updates snapshot files to reflect intentional UI changes.

Snapshot testing is best described as:

A Validating backend API responses

B Testing app performance under load

C Testing user interactions manually

D Checking if UI output matches a saved version

Snapshot testing checks if the UI output matches a saved snapshot to catch unexpected changes.

What tool is commonly used for snapshot testing in React Native?

A Jest

B Redux

C Expo

D Firebase

Jest is the testing framework commonly used for snapshot testing in React Native.

If a snapshot test fails unexpectedly, what should you do first?

A Ignore the failure

B Delete the snapshot file

C Review the UI changes to confirm if they are intentional

D Restart the device

You should review the UI changes to confirm if they are intentional before updating snapshots.

Explain how snapshot testing helps maintain UI consistency in React Native apps.

Think about how saving and comparing UI output can catch bugs.

Describe the steps to write and update a snapshot test for a React Native component.

Focus on the test code and commands used.

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