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Building with Emscripten

Setting up Emscripten#

To setup Emscripten for building Hermes, we recommend using emsdk, which is the same way Emscripten recommends for most circumstances. Follow the directions on the Emscripten website for emsdk to download the SDK.

emsdk install latest
emsdk activate latest
source ./

If you install emsdk at ~/emsdk and activate latest, then you should use this shell variable for the rest of these instructions:

$EmscriptenRoot = ~/emsdk/upstream/emscripten

If you are using the old fastcomp instead, replace upstream in the above instruction with fastcomp.

WARNING: The old fastcomp backend was removed in emscripten 2.0.0 (August 2020)

Setting up Workspace and Host Hermesc#

Hermes now requires a two stage build proecess because the VM now contains Hermes bytecode which needs to be compiled by Hermes.

Please follow the Cross Compilation to set up a workplace and build a host hermesc at $HERMES_WS_DIR/build_host_hermesc.

Building Hermes With

# Configure the build. Here the build is output to a
# directory starting with the prefix "embuild".
python3 ${HERMES_WS_DIR}/hermes/utils/build/ \
--cmake-flags " -DIMPORT_HERMESC:PATH=${HERMES_WS_DIR}/build_host_hermesc/ImportHermesc.cmake " \
--distribute \
--wasm \
--emscripten-platform=upstream \
--emscripten-root="${EmscriptenRoot?}" \
# Build Hermes. The build directory name will depend on the flags passed to
cmake --build /tmp/embuild --target hermes
# Execute hermes
node /tmp/embuild/bin/hermes.js --help

Make sure that the --emscripten-platform option matches the directory given to --emscripten-root, and is also the current activated Emscripten toolchain via emsdk activate.

See --help for more build options.

Build with CMake directly#

The script runs CMake for you with options chosen by the Hermes project. If you want to customize your build, you can take this command as a base.

cmake ${HERMES_WS_DIR}/hermes \
-B embuild \
-DCMAKE_TOOLCHAIN_FILE=${EmscriptenRoot?}/cmake/Modules/Platform/Emscripten.cmake \

Each option is explained below:

  • CMAKE_BUILD_TYPE: set it to one of CMake's build modes: Debug, Release, MinSizeRel, etc.
  • EMSCRIPTEN_FASTCOMP: set to 1 if using fastcomp, or 0 if using upstream (LLVM)
  • WASM: whether to use asm.js (0), WebAssembly (1), or both (2)
  • NODERAWFS: set to 1 if you will be running Hermes directly with Node. It enables direct access to the filesystem.
  • ALLOW_MEMORY_GROWTH: whether to pre-allocate all memory, or let it grow over time

You can customize the build generator by passing the -G option to CMake, for example -G Ninja.

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