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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 Emscripten and CMake

cmake -S ${HermesSourcePath?} -B build \
-DCMAKE_TOOLCHAIN_FILE=${EmscriptenRoot?}/emscripten/cmake/Modules/Platform/Emscripten.cmake \
# Build Hermes
cmake --build ./build --target hermes --parallel
# Execute hermes
node bin/hermes.js --help

In the commands above, replace ${HermesSourcePath?} with the path where you cloned Hermes, and ${EmscriptenRoot?} with the path to your Emscripten install.

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.