explaingit

fisker/hermes

Analysis updated 2026-07-18 · repo last pushed 2025-05-26

Audience · developerComplexity · 3/5StaleSetup · easy

TLDR

A JavaScript engine tuned for React Native apps that pre-compiles code into compact bytecode so mobile apps start up faster and use less memory.

Mindmap

mindmap
  root((hermes))
    What it does
      Runs JavaScript on mobile
      Precompiles to bytecode
      Speeds up app startup
    Tech stack
      React Native
      Bytecode compiler
    Use cases
      Faster app launch
      Lower memory use
      Custom engine builds
    Audience
      Mobile app developers
      React Native teams
    Tradeoffs
      Version matching required
      Tight React Native coupling

Code map

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filefunction / class

What do people build with it?

USE CASE 1

Speed up a React Native app's startup time by enabling Hermes in project settings.

USE CASE 2

Reduce memory usage on lower-end phones running a React Native app.

USE CASE 3

Build a custom version of the engine locally to contribute improvements.

USE CASE 4

Debug startup performance issues tied to JavaScript parsing time.

What is it built with?

React Native

How does it compare?

fisker/hermes0verflowme/alarm-clock0verflowme/seclists
Language—CSS—
Last pushed2025-05-262022-10-032020-05-03
MaintenanceStaleDormantDormant
Setup difficultyeasyeasyeasy
Complexity3/52/51/5
Audiencedevelopervibe coderops devops

Figures from each repo's GitHub metadata at analysis time.

How do you get it running?

Difficulty · easy Time to first run · 5min

Enabled via a simple config flag for most users, but Hermes and React Native versions must be matched carefully.

Copy-paste prompts

Prompt 1
Walk me through enabling Hermes in my React Native project's configuration.
Prompt 2
What version of Hermes should I use with my current React Native version to avoid crashes?
Prompt 3
Explain how Hermes's bytecode precompilation makes app startup faster than plain JavaScript.
Prompt 4
Help me clone and build Hermes locally so I can test a custom change.

Frequently asked questions

What is hermes?

A JavaScript engine tuned for React Native apps that pre-compiles code into compact bytecode so mobile apps start up faster and use less memory.

Is hermes actively maintained?

Stale — no commits in 1-2 years (last push 2025-05-26).

How hard is hermes to set up?

Setup difficulty is rated easy, with roughly 5min to a first successful run.

Who is hermes for?

Mainly developer.

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