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What is hermes?

facebook/hermes — explained in plain English

Analysis updated 2026-06-24

★ 11,043JavaScriptAudience · developerComplexity · 4/5LicenseSetup · hard

In one sentence

Hermes is a JavaScript engine built by Facebook that compiles JavaScript ahead of time so React Native mobile apps start up faster and take up less space.

Mindmap

mindmap
  root((hermes))
    What it does
      Ahead-of-time compile
      Compact bytecode
      Fast app startup
    Tech stack
      C plus plus engine
      JavaScript input
      React Native target
    Use cases
      Custom engine build
      React Native integration
      Engine research
    Audience
      Engine developers
      React Native devs
      OSS contributors
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Code map

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filefunction / class

What do people build with it?

USE CASE 1

Build Hermes from source to customize or study the JavaScript engine that powers React Native

USE CASE 2

Integrate a custom-built Hermes binary into a React Native app instead of the default bundled version

USE CASE 3

Study how ahead-of-time JavaScript compilation works compared to a standard just-in-time engine

What is it built with?

JavaScriptC++React Native

How does it compare?

facebook/hermesjaredpalmer/razzleoldboyxx/jira_clone
Stars11,04311,04811,058
LanguageJavaScriptJavaScriptJavaScript
Setup difficultyhardmoderatemoderate
Complexity4/53/53/5
Audiencedeveloperdeveloperdeveloper

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

How do you get it running?

Difficulty · hard Time to first run · 1day+

Building from source requires a C++ toolchain and platform-specific dependencies on macOS, Linux, or Windows.

Use freely for any purpose, including commercial use, as long as you keep the copyright notice.

So what is it?

Hermes is a JavaScript engine built by Facebook specifically to make React Native apps start up faster on mobile devices. A JavaScript engine is the piece of software that reads and runs JavaScript code. Most phones and browsers come with their own, but Hermes is a custom one tuned for the particular needs of React Native. The key idea behind Hermes is that it compiles JavaScript ahead of time, before the app even runs, rather than doing that work while the user waits. This makes apps open more quickly. It also produces compact bytecode, which is a compressed form of the code that takes up less space and loads faster. If you are building a React Native app and just want to use Hermes, you do not need this repository directly. You enable it through React Native's own settings. This repository is for developers who want to build Hermes from its source code, modify it, or integrate a custom-built version into their app. The README includes short build instructions for macOS, Linux, and Windows, plus links to more detailed documentation covering additional build options and how to connect a custom Hermes build to a React Native project. The project is MIT licensed and accepts outside contributions.

Copy-paste prompts

Prompt 1
Show me how to build Hermes from source on macOS and connect it to an existing React Native project
Prompt 2
Walk me through enabling a custom-built Hermes version in a React Native app on Android
Prompt 3
How does Hermes compile JavaScript ahead of time and why does it make app startup faster than a JIT engine?
Prompt 4
Show me how to run the Hermes test suite on Linux after building from source
Prompt 5
What build options does Hermes expose and when would I override the defaults?

Frequently asked questions

What is hermes?

Hermes is a JavaScript engine built by Facebook that compiles JavaScript ahead of time so React Native mobile apps start up faster and take up less space.

What language is hermes written in?

Mainly JavaScript. The stack also includes JavaScript, C++, React Native.

What license does hermes use?

Use freely for any purpose, including commercial use, as long as you keep the copyright notice.

How hard is hermes to set up?

Setup difficulty is rated hard, with roughly 1day+ to a first successful run.

Who is hermes for?

Mainly developer.

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