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Building and Running Hermes

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.

Dependencies

Hermes is a C++11 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 Mac via Homebrew:

brew install cmake git ninja

Building on Linux and macOS

Hermes will place its build files in a directory given by the HERMES_WS_DIR environment variable. Note that Hermes will download and build LLVM as part of its own build.

Create a base directory to work in, e.g. ~/workspace, and cd into it. Follow the steps below to build LLVM and generate the Hermes build system:

export HERMES_WS_DIR="$PWD"
git clone git@github.com:facebook/hermes.git
hermes/utils/build_llvm.py
hermes/utils/configure.sh

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

cd build && ninja

Building on Windows

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

export HERMES_WS_DIR='c:/ws'
cd "$HERMES_WS_DIR"
git -c core.autocrlf=false clone git@github.com:facebook/hermes.git
BUILD_SYSTEM='Visual Studio 16 2019' CMAKE_FLAGS='-A x64' DISTRIBUTE=1 hermes/utils/build_llvm.py
CMAKE_FLAGS='-A x64 -DLLVM_ENABLE_LTO=OFF' DISTRIBUTE=1 hermes/utils/configure.sh 'Visual Studio 16 2019'
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.

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

Release Build

The above instructions create an unoptimized debug build. The DISTRIBUTE=1 environment variable will enable a release build, in the build_release directory. Example:

env DISTRIBUTE=1 hermes/utils/build_llvm.py
env DISTRIBUTE=1 hermes/utils/configure.sh
cd build_release && ninja

Other Tools

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

  • hdb: JavaScript command line debugger
  • hermes-repl: JavaScript interactive REPL
  • 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.