Hermes engine in React Native - UI Render Trace

Component - Hermes engine

The Hermes engine is a JavaScript engine optimized for React Native apps. It helps the app start faster and run smoother by efficiently running JavaScript code on mobile devices.

Widget Tree

App
├─ View (Container)
│  ├─ Text (Title)
│  ├─ Text (Description)
│  └─ Button (Toggle Hermes)
└─ View (Status)
   └─ Text (Hermes Status)

The main App component contains a container View with a title Text, a description Text, and a Button to toggle Hermes engine usage. Below, a status View shows the current Hermes engine status with a Text component.

Render Trace - 7 Steps

Step 1: App

Step 2: View (Container)

Step 3: Text (Title)

Step 4: Text (Description)

Step 5: Button (Toggle Hermes)

Step 6: View (Status)

Step 7: Text (Hermes Status)

State Change - Re-render

Trigger:User taps the 'Toggle Hermes' button

Before

Hermes engine is enabled

→

After

Hermes engine is disabled

Re-renders:App component and all its children re-render to update the status text and button state

UI Quiz - 3 Questions

What does the 'Toggle Hermes' button do in this UI?

A Reloads the app screen

B Changes the app theme color

C Switches Hermes engine on or off

D Opens app settings

Key Insight

Using the Hermes engine in React Native improves app speed by running JavaScript more efficiently. The UI clearly shows the engine status and lets users toggle it, demonstrating how state changes update the screen smoothly.