Hermes engine in React Native - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the Hermes engine in React Native?

Hermes is a JavaScript engine optimized for running React Native apps on Android and iOS. It improves app start time, reduces memory usage, and enhances overall performance.

intermediate

How does Hermes improve app startup time?

Hermes compiles JavaScript into bytecode ahead of time, so the app can start faster by skipping the usual JavaScript parsing and compilation during launch.

beginner

Which platforms support Hermes in React Native?

Hermes supports Android and iOS platforms for React Native apps, helping improve performance on both mobile operating systems.

intermediate

How do you enable Hermes in a React Native project?

To enable Hermes, you update the React Native configuration files (like android/app/build.gradle for Android and Podfile for iOS) to turn Hermes on, then rebuild the app.

advanced

What are some trade-offs when using Hermes?

While Hermes improves startup and memory, it may have limited support for some JavaScript features and debugging tools compared to other engines like JavaScriptCore.

What is the main benefit of using Hermes in React Native?

A Built-in database support

B Better UI design tools

C Automatic code generation

D Faster app startup and lower memory use

Hermes is designed to improve app startup speed and reduce memory consumption.

Which platforms can run React Native apps with Hermes?

A Only Android

B Only iOS

C Android and iOS

D Windows and Linux

Hermes supports both Android and iOS platforms.

How does Hermes improve performance?

A By compiling JavaScript to bytecode ahead of time

B By using more CPU cores

C By caching images automatically

D By reducing app size only

Hermes compiles JavaScript into bytecode before running, speeding up app launch.

What must you do to use Hermes in your React Native app?

A Enable Hermes in build configuration and rebuild the app

B Install a separate Hermes app

C Write JavaScript in a special Hermes syntax

D Use a different programming language

Hermes is enabled by changing build settings and rebuilding the app.

What is a possible downside of using Hermes?

A Slower app startup

B Limited support for some JavaScript features

C Increased app size

D No support for Android

Hermes may not support all JavaScript features or debugging tools fully.

Explain what Hermes engine is and why it is used in React Native apps.

Think about how apps start and run faster with Hermes.

Describe the steps to enable Hermes in a React Native project.

Focus on what files you change and what you do after.

Practice

1. What is the main purpose of the Hermes engine in React Native?

easy

A. To add new UI components automatically

B. To make apps start faster and use less memory

C. To convert JavaScript code to native Android code

D. To manage app navigation between screens

Solution

1. Step 1: Understand Hermes role

   Hermes is a JavaScript engine designed to improve app performance.

2. Step 2: Identify its benefits

   It helps apps start faster and reduces memory and battery use.

3. Final Answer:

   To make apps start faster and use less memory -> Option B

4. Quick Check:

   Hermes improves app speed and memory use = A [OK]

Hint: Hermes speeds up app start and saves memory [OK]

Common Mistakes:

• Thinking Hermes adds UI components
• Confusing Hermes with native code compiler
• Believing Hermes manages navigation

2. Which of the following is the correct way to enable Hermes in a React Native Android project?

easy

A. Add hermesEnabled = true in index.js

B. Import Hermes from 'react-native-hermes' in App.js

C. Set enableHermes: true in android/app/build.gradle

D. Set useHermes = true in package.json

Solution

1. Step 1: Locate Hermes enabling method

   Hermes is enabled in the Android build config file android/app/build.gradle.

2. Step 2: Confirm correct syntax

   The correct setting is enableHermes: true inside the project.ext.react block.

3. Final Answer:

   Set enableHermes: true in android/app/build.gradle -> Option C

4. Quick Check:

   Enable Hermes in build.gradle = B [OK]

Hint: Enable Hermes in android/app/build.gradle [OK]

Common Mistakes:

• Trying to enable Hermes in JavaScript files
• Using wrong property names like useHermes
• Adding Hermes import statements

3. Given this React Native code snippet running with Hermes enabled, what is the expected behavior?

console.log('App started');
setTimeout(() => console.log('Timeout done'), 0);
console.log('End of script');

medium

A. Logs: 'App started', 'End of script', 'Timeout done'

B. Logs: 'End of script', 'App started', 'Timeout done'

C. Logs: 'Timeout done', 'App started', 'End of script'

D. Logs: 'App started', 'Timeout done', 'End of script'

Solution

1. Step 1: Understand JavaScript event loop with Hermes

   Even with Hermes, JavaScript runs synchronously first, then asynchronous callbacks.

2. Step 2: Trace the console logs order

   First logs 'App started', then 'End of script', then the setTimeout callback logs 'Timeout done'.

3. Final Answer:

   Logs: 'App started', 'End of script', 'Timeout done' -> Option A

4. Quick Check:

   Sync logs before async timeout = D [OK]

Hint: setTimeout callback runs after synchronous code [OK]

Common Mistakes:

• Assuming setTimeout runs immediately
• Confusing Hermes behavior with normal JS
• Expecting logs in wrong order

4. You enabled Hermes in your React Native app but it crashes on startup. Which of these is the most likely cause?

medium

A. You forgot to clean and rebuild the project after enabling Hermes

B. You did not import Hermes in your JavaScript files

C. You set enableHermes: false in build config

D. You used unsupported React Native components

Solution

1. Step 1: Identify common Hermes setup errors

   After enabling Hermes, the project must be cleaned and rebuilt to apply changes properly.

2. Step 2: Understand why skipping rebuild causes crashes

   Without rebuild, old native binaries conflict with Hermes causing startup crashes.

3. Final Answer:

   You forgot to clean and rebuild the project after enabling Hermes -> Option A

4. Quick Check:

   Clean and rebuild fixes Hermes crashes = A [OK]

Hint: Always clean and rebuild after enabling Hermes [OK]

Common Mistakes:

• Thinking Hermes needs JS imports
• Ignoring rebuild step
• Blaming unrelated components

5. You want to reduce your React Native app's memory usage on Android. How can Hermes help, and what must you do to enable it properly?

hard

A. Hermes reduces memory by lazy loading components; enable it by adding hermesLazyLoad: true in package.json

B. Hermes reduces memory by compressing images; enable it by importing Hermes in App.js

C. Hermes reduces memory by caching network data; enable it by setting cacheHermes: true in android/app/build.gradle

D. Hermes reduces memory by optimizing JS execution; enable it by setting enableHermes: true and rebuilding the app

Solution

1. Step 1: Understand Hermes memory optimization

   Hermes optimizes JavaScript execution to use less memory and battery.

2. Step 2: Know how to enable Hermes correctly

   Set enableHermes: true in android/app/build.gradle and rebuild the app to apply changes.

3. Final Answer:

   Hermes reduces memory by optimizing JS execution; enable it by setting enableHermes: true and rebuilding the app -> Option D

4. Quick Check:

   Enable Hermes in build config and rebuild = C [OK]

Hint: Enable Hermes in build config and rebuild to save memory [OK]

Common Mistakes:

• Thinking Hermes compresses images
• Trying to enable Hermes via JS imports
• Using wrong config keys like cacheHermes