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Building and Running

This document describes how to build and run Hermes as a standalone compiler and VM. To use Hermes in the context of a React Native app, see the React Native documentation.


Hermes is a C++14 project. clang, gcc, and Visual C++ are supported. Hermes also requires cmake, git, ICU, Python, and zip. It builds with CMake and ninja.

The Hermes REPL will also use libreadline, if available.

To install dependencies on Ubuntu:

apt install cmake git ninja-build libicu-dev python zip libreadline-dev

On Arch Linux:

pacman -S cmake git ninja icu python zip readline

On Mac via Homebrew:

brew install cmake git ninja

Building on Linux and macOS#

Hermes will place its build files in the current directory by default. You can also give explicit source and build directories, use --help on the build scripts to see how.

Create a base directory to work in, e.g. ~/workspace, and cd into it. (Tip: avoid naming it hermes, as hermes will be one of several subdirectories in the workspace). After cding, follow the steps below to generate the Hermes build system:

git clone

The build system has now been generated in the build directory. To perform the build:

cd build && ninja

Release Build#

The above instructions create an unoptimized debug build. The --distribute flag will enable a release build, in the build_release directory. Example:

hermes/utils/build/ --distribute
cd build_release && ninja

Building on Windows#

The Windows build depends on which particular combination of GitBash/Cygwin/WSL and Visual Studio is used.

git -c core.autocrlf=false clone
hermes/utils/build/ --build-system='Visual Studio 16 2019' --distribute
cd build_release && MSBuild.exe ALL_BUILD.vcxproj /p:Configuration=Release

Running Hermes#

The primary binary is the hermes tool, which will be found at build/bin/hermes. This tool compiles JavaScript to Hermes bytecode. It can also execute JavaScript, from source or bytecode or be used as a REPL.

Executing JavaScript with Hermes#

hermes test.js

Compiling and Executing JavaScript with Bytecode#

hermes -emit-binary -out test.hbc test.js
hermes test.hbc

Running Tests#

To run the Hermes test suite:

ninja check-hermes

Formatting Code#

To automatically format all your changes, you will need clang-format, then simply run:


Other Tools#

In addition to hermes, the following tools will be built:

  • hdb: JavaScript command line debugger
  • hbcdump: Hermes bytecode disassembler
  • hermesc: Standalone Hermes compiler. This can compile JavaScript to Hermes bytecode, but does not support executing it.
  • hvm: Standalone Hermes VM. This can execute Hermes bytecode, but does not support compiling it.
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