Why Hermes engine in React Native? - Purpose & Use Cases

The Big Idea

What if your app could start instantly and run smoothly on any phone without extra coding?

The Scenario

Imagine building a React Native app that feels slow and clunky on older phones. You try to make it faster by tweaking JavaScript code, but the app still takes a long time to start and uses too much battery.

The Problem

Without a specialized engine, JavaScript runs slower because it must be interpreted every time. This causes delays in app startup and makes animations lag. It also drains battery faster, frustrating users and making your app look unprofessional.

The Solution

Hermes is a JavaScript engine optimized for React Native. It compiles JavaScript ahead of time, so the app starts faster and runs smoother. It reduces memory use and battery drain, giving users a better experience without extra work from you.

Before vs After

✗ Before

import React from 'react';
import { Text } from 'react-native';
// App runs with default JS engine, slower startup
export default function App() {
  return <Text>Hello</Text>;
}

✓ After

import React from 'react';
import { Text } from 'react-native';
// Enable Hermes engine in React Native config for faster startup
export default function App() {
  return <Text>Hello</Text>;
}

What It Enables

With Hermes, your React Native apps launch quickly and run smoothly even on low-end devices, delighting users everywhere.

Real Life Example

Think of a social media app that loads instantly when opened, with smooth scrolling and less battery drain—Hermes makes this possible by optimizing JavaScript execution behind the scenes.

Key Takeaways

Running JavaScript without optimization causes slow app startup and lag.

Hermes compiles JavaScript ahead of time for faster, smoother apps.

Using Hermes improves user experience on all devices, especially older ones.

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