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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
cmake -S hermes -B build -G Ninja

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

cmake --build ./build

Release Build

The above instructions create an unoptimized debug build. The -DCMAKE_BUILD_TYPE=Release flag will create a release build:

cmake -S hermes -B build_release -G Ninja -DCMAKE_BUILD_TYPE=Release
cmake --build ./build_release

Building on Windows

To build on Windows using Visual Studio with a checkout in the hermes directory:

cmake -S hermes -B build -G 'Visual Studio 16 2019'
cmake --build ./build

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:

cmake --build ./build --target check-hermes

To run Hermes against the test262 suite, you need to have a Hermes binary built already and a clone of the test262 repo:

hermes/utils/testsuite/ -b <hermes_build> <test262>

E.g. if we configured at ~/hermes_build (i.e. ~/hermes_build/bin/hermes is an executable) and cloned test262 at ~/test262, then perform:

hermes/utils/testsuite/ -b ~/hermes_build ~/test262/test

Note that you can also only test against part of a test suite, e.g. to run the Intl402 subset of the test262, you can specifiy a subdir:

hermes/utils/testsuite/ -b ~/hermes_build ~/test262/test/intl402

Formatting Code

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


AddressSanitizer (ASan) Build

The -HERMES_ENABLE_ADDRESS_SANITIZER=ON flag will create a ASan build:

git clone
cmake -S hermes -B asan_build -G Ninja -D HERMES_ENABLE_ADDRESS_SANITIZER=ON
cmake --build ./asan_build

You can verify the build by looking for asan symbols in the hermes binary:

nm asan_build/bin/hermes | grep asan

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.