emnapi

Sponsors

Node-API implementation for Emscripten, wasi-sdk and clang with wasm support.

This project aims to

• Help users port their or existing Node-API native addons to wasm with code change as less as possible.
• Make runtime behavior matches native Node.js as much as possible.

This project also powers the WebAssembly feature for napi-rs, and enables many Node.js native addons to run on StackBlitz's WebContainer.

Node-API changes will be synchronized into this repo.

See documentation for more details:

• https://toyobayashi.github.io/emnapi-docs/guide/
• https://emnapi-docs.vercel.app/guide/

中文文档：

• https://toyobayashi.github.io/emnapi-docs/zh/guide/
• https://emnapi-docs.vercel.app/zh/guide/

Full API List

How to build Node-API official examples

Prerequests

You will need to install:

• Node.js >= v16.15.0
• npm >= v8
• Emscripten >= v3.1.9 / wasi-sdk / LLVM clang with wasm support
• (Optional) CMake >= v3.13
• (Optional) node-gyp >= v10.2.0
• (Optional) ninja
• (Optional) make
• (Optional) node-addon-api >= 6.1.0

There are several choices to get make for Windows user

• Install mingw-w64, then use mingw32-make
• Download MSVC prebuilt binary of GNU make, add to %Path% then rename it to mingw32-make
• Install Visual Studio 2022 C++ desktop workload, use nmake in Visual Studio Developer Command Prompt
• Install Visual C++ Build Tools, use nmake in Visual Studio Developer Command Prompt

Verify your environment:

node -v
npm -v
emcc -v

# clang -v
# clang -print-targets # ensure wasm32 target exists

cmake --version

# if you use node-gyp
node-gyp --version

# if you use ninja
ninja --version

# if you use make
make -v

# if you use nmake in Visual Studio Developer Command Prompt
nmake /?

Build from source

You need to set EMSDK and WASI_SDK_PATH environment variables.

git clone https://github.com/toyobayashi/emnapi.git
cd ./emnapi
npm install -g node-gyp
npm install
npm run build             # output ./packages/*/dist
node ./script/release.js  # output ./out

# test
npm run rebuild:test
npm test

See CONTRIBUTING for more details.

Quick Start

NPM Install

npm install -D emnapi
npm install @emnapi/runtime

# for non-emscripten
npm install @emnapi/core

# if you use node-addon-api
npm install node-addon-api

Each package should match the same version.

Using C

Create hello.c.

#include <node_api.h>

#define NODE_API_CALL(env, the_call)                            \
  do {                                                          \
    if ((the_call) != napi_ok) {                                \
      const napi_extended_error_info *error_info;               \
      napi_get_last_error_info((env), &error_info);             \
      bool is_pending;                                          \
      const char* err_message = error_info->error_message;      \
      napi_is_exception_pending((env), &is_pending);            \
      if (!is_pending) {                                        \
        const char* error_message = err_message != NULL ?       \
          err_message :                                         \
          "empty error message";                                \
        napi_throw_error((env), NULL, error_message);           \
      }                                                         \
      return NULL;                                              \
    }                                                           \
  } while (0)

static napi_value js_hello(napi_env env, napi_callback_info info) {
  napi_value world;
  const char* str = "world";
  NODE_API_CALL(env, napi_create_string_utf8(env, str, NAPI_AUTO_LENGTH, &world));
  return world;
}

NAPI_MODULE_INIT() {
  napi_value hello;
  NODE_API_CALL(env, napi_create_function(env, "hello", NAPI_AUTO_LENGTH,
                                      js_hello, NULL, &hello));
  NODE_API_CALL(env, napi_set_named_property(env, exports, "hello", hello));
  return exports;
}

The C code is equivalant to the following JavaScript:

module.exports = (function (exports) {
  const hello = function hello () {
    // native code in js_hello
    const world = 'world'
    return world
  }

  exports.hello = hello
  return exports
})(module.exports)

Building

emcc -O3 \
     -DBUILDING_NODE_EXTENSION \
     "-DNAPI_EXTERN=__attribute__((__import_module__(\"env\")))" \
     -I./node_modules/emnapi/include/node \
     -L./node_modules/emnapi/lib/wasm32-emscripten \
     --js-library=./node_modules/emnapi/dist/library_napi.js \
     -sEXPORTED_FUNCTIONS="['_malloc','_free','_napi_register_wasm_v1','_node_api_module_get_api_version_v1']" \
     -o hello.js \
     hello.c \
     -lemnapi

clang -O3 \
      -DBUILDING_NODE_EXTENSION \
      -I./node_modules/emnapi/include/node \
      -L./node_modules/emnapi/lib/wasm32-wasi \
      --target=wasm32-wasi \
      --sysroot=$WASI_SDK_PATH/share/wasi-sysroot \
      -mexec-model=reactor \
      -Wl,--initial-memory=16777216 \
      -Wl,--export-dynamic \
      -Wl,--export=malloc \
      -Wl,--export=free \
      -Wl,--export=napi_register_wasm_v1 \
      -Wl,--export-if-defined=node_api_module_get_api_version_v1 \
      -Wl,--import-undefined \
      -Wl,--export-table \
      -o hello.wasm \
      hello.c \
      -lemnapi

Choose libdlmalloc.a or libemmalloc.a for malloc and free.

clang -O3 \
      -DBUILDING_NODE_EXTENSION \
      -I./node_modules/emnapi/include/node \
      -L./node_modules/emnapi/lib/wasm32 \
      --target=wasm32 \
      -nostdlib \
      -Wl,--no-entry \
      -Wl,--initial-memory=16777216 \
      -Wl,--export-dynamic \
      -Wl,--export=malloc \
      -Wl,--export=free \
      -Wl,--export=napi_register_wasm_v1 \
      -Wl,--export-if-defined=node_api_module_get_api_version_v1 \
      -Wl,--import-undefined \
      -Wl,--export-table \
      -o hello.wasm \
      hello.c \
      -lemnapi \
      -ldlmalloc # -lemmalloc

Initialization

To initialize emnapi, you need to import the emnapi runtime to create a Context by createContext or getDefaultContext first. Each context owns isolated Node-API object such as napi_env, napi_value, napi_ref. If you have multiple emnapi modules, you should reuse the same Context across them.

declare namespace emnapi {
  // module '@emnapi/runtime'
  export class Context { /* ... */ }
  /** Create a new context */
  export function createContext (): Context
  /** Create or get */
  export function getDefaultContext (): Context
  // ...
}

then call Module.emnapiInit after emscripten runtime initialized. Module.emnapiInit only do initialization once, it will always return the same binding exports after successfully initialized.

declare namespace Module {
  interface EmnapiInitOptions {
    context: emnapi.Context

    /** node_api_get_module_file_name */
    filename?: string

    /**
     * Support following async_hooks related things
     * on Node.js runtime only
     * 
     * napi_async_init,
     * napi_async_destroy,
     * napi_make_callback,
     * async resource parameter of
     * napi_create_async_work and napi_create_threadsafe_function
     */
    nodeBinding?: typeof import('@emnapi/node-binding')

    /** See Multithread part */
    asyncWorkPoolSize?: number
  }
  export function emnapiInit (options: EmnapiInitOptions): any
}

<script src="node_modules/@emnapi/runtime/dist/emnapi.min.js"></script>
<script src="hello.js"></script>
<script>
Module.onRuntimeInitialized = function () {
  var binding;
  try {
    binding = Module.emnapiInit({ context: emnapi.getDefaultContext() });
  } catch (err) {
    console.error(err);
    return;
  }
  var msg = 'hello ' + binding.hello();
  window.alert(msg);
};

// if -sMODULARIZE=1
Module({ /* Emscripten module init options */ }).then(function (Module) {
  var binding = Module.emnapiInit({ context: emnapi.getDefaultContext() });
});
</script>

If you are using Visual Studio Code and have Live Server extension installed, you can right click the HTML file in Visual Studio Code source tree and click Open With Live Server, then you can see the hello world alert!

Running on Node.js:

const emnapi = require('@emnapi/runtime')
const Module = require('./hello.js')

Module.onRuntimeInitialized = function () {
  let binding
  try {
    binding = Module.emnapiInit({ context: emnapi.getDefaultContext() })
  } catch (err) {
    console.error(err)
    return
  }
  const msg = `hello ${binding.hello()}`
  console.log(msg)
}

// if -sMODULARIZE=1
Module({ /* Emscripten module init options */ }).then((Module) => {
  const binding = Module.emnapiInit({ context: emnapi.getDefaultContext() })
})

For non-emscripten, you need to use @emnapi/core. The initialization is similar to emscripten.

<script src="node_modules/@emnapi/runtime/dist/emnapi.min.js"></script>
<script src="node_modules/@emnapi/core/dist/emnapi-core.min.js"></script>
<script>
emnapiCore.instantiateNapiModule(fetch('./hello.wasm'), {
  context: emnapi.getDefaultContext(),
  overwriteImports (importObject) {
    // importObject.env = {
    //   ...importObject.env,
    //   ...importObject.napi,
    //   ...importObject.emnapi,
    //   // ...
    // }
  }
}).then(({ instance, module, napiModule }) => {
  const binding = napiModule.exports
  // ...
})
</script>

Using WASI on Node.js

const { instantiateNapiModule } = require('@emnapi/core')
const { getDefaultContext } = require('@emnapi/runtime')
const { WASI } = require('wasi')
const fs = require('fs')

instantiateNapiModule(fs.promises.readFile('./hello.wasm'), {
  wasi: new WASI({ /* ... */ }),
  context: getDefaultContext(),
  overwriteImports (importObject) {
    // importObject.env = {
    //   ...importObject.env,
    //   ...importObject.napi,
    //   ...importObject.emnapi,
    //   // ...
    // }
  }
}).then(({ instance, module, napiModule }) => {
  const binding = napiModule.exports
  // ...
})

Using WASI on browser, you can use WASI polyfill in wasm-util, and memfs-browser

import { instantiateNapiModule } from '@emnapi/core'
import { getDefaultContext } from '@emnapi/runtime'
import { WASI } from '@tybys/wasm-util'
import { Volume, createFsFromVolume } from 'memfs-browser'

const fs = createFsFromVolume(Volume.fromJSON({ /* ... */ }))
instantiateNapiModule(fetch('./hello.wasm'), {
  wasi: new WASI({ fs, /* ... */ })
  context: getDefaultContext(),
  overwriteImports (importObject) {
    // importObject.env = {
    //   ...importObject.env,
    //   ...importObject.napi,
    //   ...importObject.emnapi,
    //   // ...
    // }
  }
}).then(({ instance, module, napiModule }) => {
  const binding = napiModule.exports
  // ...
})

Using C++ and node-addon-api

Require node-addon-api >= 6.1.0

npm install node-addon-api

Note: C++ wrapper can only be used to target Node.js v14.6.0+ and modern browsers those support FinalizationRegistry and WeakRef (v8 engine v8.4+)!

Create hello.cpp.

#include <napi.h>

Napi::String Method(const Napi::CallbackInfo& info) {
  Napi::Env env = info.Env();
  return Napi::String::New(env, "world");
}

Napi::Object Init(Napi::Env env, Napi::Object exports) {
  exports.Set(Napi::String::New(env, "hello"),
              Napi::Function::New(env, Method)).Check();
  return exports;
}

NODE_API_MODULE(NODE_GYP_MODULE_NAME, Init)

Compile hello.cpp using em++. C++ exception is disabled by Emscripten default, and not supported by wasi-sdk, so predefine -DNAPI_DISABLE_CPP_EXCEPTIONS and -DNODE_ADDON_API_ENABLE_MAYBE here. If you would like to enable C++ exception, use -sDISABLE_EXCEPTION_CATCHING=0 instead and remove .Check() call. See official documentation here.

Building

em++ -O3 \
     -DBUILDING_NODE_EXTENSION \
     "-DNAPI_EXTERN=__attribute__((__import_module__(\"env\")))" \
     -DNAPI_DISABLE_CPP_EXCEPTIONS \
     -DNODE_ADDON_API_ENABLE_MAYBE \
     -I./node_modules/emnapi/include/node \
     -I./node_modules/node-addon-api \
     -L./node_modules/emnapi/lib/wasm32-emscripten \
     --js-library=./node_modules/emnapi/dist/library_napi.js \
     -sEXPORTED_FUNCTIONS="['_malloc','_free','_napi_register_wasm_v1','_node_api_module_get_api_version_v1']" \
     -o hello.js \
     hello.cpp \
     -lemnapi

clang++ -O3 \
        -DBUILDING_NODE_EXTENSION \
        -DNAPI_DISABLE_CPP_EXCEPTIONS \
        -DNODE_ADDON_API_ENABLE_MAYBE \
        -I./node_modules/emnapi/include/node \
        -I./node_modules/node-addon-api \
        -L./node_modules/emnapi/lib/wasm32-wasi \
        --target=wasm32-wasi \
        --sysroot=$WASI_SDK_PATH/share/wasi-sysroot \
        -fno-exceptions \
        -mexec-model=reactor \
        -Wl,--initial-memory=16777216 \
        -Wl,--export-dynamic \
        -Wl,--export=malloc \
        -Wl,--export=free \
        -Wl,--export=napi_register_wasm_v1 \
        -Wl,--export-if-defined=node_api_module_get_api_version_v1 \
        -Wl,--import-undefined \
        -Wl,--export-table \
        -o hello.wasm \
        hello.cpp \
        -lemnapi

node-addon-api is using the C++ standard libraries, so you must use WASI if you are using node-addon-api.

You can still use wasm32-unknown-unknown target if you use Node-API C API only in C++.

clang++ -O3 \
        -DBUILDING_NODE_EXTENSION \
        -I./node_modules/emnapi/include/node \
        -L./node_modules/emnapi/lib/wasm32 \
        --target=wasm32 \
        -fno-exceptions \
        -nostdlib \
        -Wl,--no-entry \
        -Wl,--initial-memory=16777216 \
        -Wl,--export-dynamic \
        -Wl,--export=malloc \
        -Wl,--export=free \
        -Wl,--export=napi_register_wasm_v1 \
        -Wl,--export-if-defined=node_api_module_get_api_version_v1 \
        -Wl,--import-undefined \
        -Wl,--export-table \
        -o node_api_c_api_only.wasm \
        node_api_c_api_only.cpp \
        -lemnapi \
        -ldlmalloc # -lemmalloc

operator new and operator delete.

#include <stddef.h>

extern "C" void* malloc(size_t size);
extern "C" void free(void* p);

void* operator new(size_t size) {
  return malloc(size);
}

void operator delete(void* p) noexcept {
  free(p);
}

Using CMake

Create CMakeLists.txt.

cmake_minimum_required(VERSION 3.13)

project(emnapiexample)

add_subdirectory("${CMAKE_CURRENT_SOURCE_DIR}/node_modules/emnapi")

add_executable(hello hello.c)

target_link_libraries(hello emnapi)
if(CMAKE_SYSTEM_NAME STREQUAL "Emscripten")
  target_link_options(hello PRIVATE
    "-sEXPORTED_FUNCTIONS=['_malloc','_free','_napi_register_wasm_v1','_node_api_module_get_api_version_v1']"
  )
elseif(CMAKE_SYSTEM_NAME STREQUAL "WASI")
  set_target_properties(hello PROPERTIES SUFFIX ".wasm")
  target_link_options(hello PRIVATE
    "-mexec-model=reactor"
    "-Wl,--export=napi_register_wasm_v1"
    "-Wl,--export-if-defined=node_api_module_get_api_version_v1"
    "-Wl,--initial-memory=16777216,--export-dynamic,--export=malloc,--export=free,--import-undefined,--export-table"
  )
elseif((CMAKE_C_COMPILER_TARGET STREQUAL "wasm32") OR (CMAKE_C_COMPILER_TARGET STREQUAL "wasm32-unknown-unknown"))
  set_target_properties(hello PROPERTIES SUFFIX ".wasm")
  target_link_options(hello PRIVATE
    "-nostdlib"
    "-Wl,--export=napi_register_wasm_v1"
    "-Wl,--export-if-defined=node_api_module_get_api_version_v1"
    "-Wl,--no-entry"
    "-Wl,--initial-memory=16777216,--export-dynamic,--export=malloc,--export=free,--import-undefined,--export-table"
  )
  target_link_libraries(hello dlmalloc)
  # target_link_libraries(hello emmalloc)
endif()

If you use node-addon-api, you can use -DEMNAPI_FIND_NODE_ADDON_API=ON or manually add node-addon-api directory to the include dir via include_directories() or target_include_directories().

mkdir build

# emscripten
emcmake cmake -DCMAKE_BUILD_TYPE=Release \
              -DEMNAPI_FIND_NODE_ADDON_API=ON \
              -G Ninja -H. -Bbuild

# wasi-sdk
cmake -DCMAKE_TOOLCHAIN_FILE=$WASI_SDK_PATH/share/cmake/wasi-sdk.cmake \
      -DWASI_SDK_PREFIX=$WASI_SDK_PATH \
      -DEMNAPI_FIND_NODE_ADDON_API=ON \
      -DCMAKE_BUILD_TYPE=Release \
      -G Ninja -H. -Bbuild

# wasm32
cmake -DCMAKE_TOOLCHAIN_FILE=node_modules/emnapi/cmake/wasm32.cmake \
      -DLLVM_PREFIX=$WASI_SDK_PATH \
      -DCMAKE_BUILD_TYPE=Release \
      -G Ninja -H. -Bbuild

cmake --build build

Output code can run in recent version modern browsers and Node.js latest LTS. IE is not supported.

Using node-gyp (Experimental)

Require node-gyp >= 10.2.0

See emnapi-node-gyp-test for examples.

• Variables

Arch: node-gyp configure --arch=<wasm32 | wasm64>

// node-gyp configure -- -Dvariable_name=value

declare var OS: 'emscripten' | 'wasi' | 'unknown' | 'wasm' | ''

/**
 * Enable async work and threadsafe-functions
 * @default 0
 */
declare var wasm_threads: 0 | 1

/** @default 1048576 */
declare var stack_size: number

/** @default 16777216 */
declare var initial_memory: number

/** @default 2147483648 */
declare var max_memory: number

/** @default path.join(path.dirname(commonGypiPath,'./dist/library_napi.js')) */
declare var emnapi_js_library: string

/** @default 0 */
declare var emnapi_manual_linking: 0 | 1

• Create binding.gyp

{
  "targets": [
    {
      "target_name": "hello",
      "sources": [
        "hello.c"
      ],
      "conditions": [
        ["OS == 'emscripten'", {
          "product_extension": "js", # required

          "cflags": [],
          "cflags_c": [],
          "cflags_cc": [],
          "ldflags": []
        }],
        ["OS == 'wasi'", {
          # ...
        }],
        ["OS in ' wasm unknown'", {
          # ...
        }]
      ]
    }
  ]
}

• Add the following environment variables.

# Linux or macOS
export GYP_CROSSCOMPILE=1

# emscripten
export AR_target="$EMSDK/upstream/emscripten/emar"
export CC_target="$EMSDK/upstream/emscripten/emcc"
export CXX_target="$EMSDK/upstream/emscripten/em++"

# wasi-sdk
export AR_target="$WASI_SDK_PATH/bin/ar"
export CC_target="$WASI_SDK_PATH/bin/clang"
export CXX_target="$WASI_SDK_PATH/bin/clang++"

@REM Windows

set GYP_CROSSCOMPILE=1

@REM emscripten
call set AR_target=%%EMSDK:\=/%%/upstream/emscripten/emar.bat
call set CC_target=%%EMSDK:\=/%%/upstream/emscripten/emcc.bat
call set CXX_target=%%EMSDK:\=/%%/upstream/emscripten/em++.bat

@REM wasi-sdk
call set AR_target=%%WASI_SDK_PATH:\=/%%/bin/ar.exe
call set CC_target=%%WASI_SDK_PATH:\=/%%/bin/clang.exe
call set CXX_target=%%WASI_SDK_PATH:\=/%%/bin/clang++.exe

• Build

# Linux or macOS

# emscripten
emmake node-gyp rebuild \
  --arch=wasm32 \
  --nodedir=./node_modules/emnapi \
  -- -f make-emscripten # -Dwasm_threads=1

# wasi
node-gyp rebuild \
  --arch=wasm32 \
  --nodedir=./node_modules/emnapi \
  -- -f make-wasi # -Dwasm_threads=1

# bare wasm32
node-gyp rebuild \
  --arch=wasm32 \
  --nodedir=./node_modules/emnapi \
  -- -f make-wasm # -Dwasm_threads=1

@REM Use make generator on Windows
@REM Run the bat file in POSIX-like environment (e.g. Cygwin)

@REM emscripten
call npx.cmd node-gyp configure --arch=wasm32 --nodedir=./node_modules/emnapi -- -f make-emscripten
call emmake.bat make -C %~dp0build

@REM wasi
call npx.cmd node-gyp configure --arch=wasm32 --nodedir=./node_modules/emnapi -- -f make-wasi
make -C %~dp0build

@REM bare wasm32
call npx.cmd node-gyp configure --arch=wasm32 --nodedir=./node_modules/emnapi -- -f make-wasm
make -C %~dp0build

Using Rust

See napi-rs

Multithread

Related API:

• napi_*_async_work
• napi_*_threadsafe_function

They are available in emnapi, but you need to know more details before you start to use them. Now emnapi has 3 implementations of async work and 2 implementations of TSFN:

• Async work
  • A. Libuv threadpool and pthread based implementation in C
  • B. Single thread mock in JavaScript
  • C. Web worker based implementation in C (stack allocation) and JavaScript
• TSFN
  • D. Libuv and pthread based implementation in C
  • E. Web worker based implementation in JavaScript

| | Library to Link | wasm32-emscripten | wasm32 | wasm32-wasi | wasm32-wasi-threads | |---|------------------------|---------------------|----------|---------------|-----------------------| | A | libemnapi-mt.a | ✅ | ❌ | ❌ | ✅ | | B | libemnapi-basic(-mt).a | ✅ | ✅ | ✅ | ✅ | | C | libemnapi-basic-mt.a | ❌ | ✅ | ❌ | ✅ | | D | libemnapi-mt.a | ✅ | ❌ | ❌ | ✅ | | E | libemnapi-basic(-mt).a | ✅ | ✅ | ✅ | ✅ |

There are some limitations on browser about wasi-libc's pthread implementation, for example pthread_mutex_lock may call __builtin_wasm_memory_atomic_wait32(memory.atomic.wait32) which is disallowed in browser JS main thread. While Emscripten's pthread implementation has considered usage in browser. If you need to run your addon with multithreaded features on browser, we recommend you use Emscripten A & D, or bare wasm32 C & E.

Note: For browsers, all the multithreaded features relying on Web Workers (Emscripten pthread also relying on Web Workers) require cross-origin isolation to enable SharedArrayBuffer. You can make a page cross-origin isolated by serving the page with these headers:

Cross-Origin-Embedder-Policy: require-corp
Cross-Origin-Opener-Policy: same-origin

If you would like to avoid SharedArrayBuffer and cross-origin isolation, please use B & E (link against libemnapi-basic.a), see the following table for more details.

About Prebuilt Libraries

Prebuilt libraries can be found in the lib directory in emnapi npm package.

| Library | Description | wasm32-emscripten | wasm32 | wasm32-wasi | wasm32-wasi-threads | |----------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------------|----------|---------------|-----------------------------------------| | libemnapi.a | no atomics feature. no libuv port. napi_*_async_work and napi_*_threadsafe_function always return napi_generic_failure. | ✅ | ✅ | ✅ | ✅ | | libemnapi-mt.a | atomics feature enabled. napi_*_async_work and napi_*_threadsafe_function are based on pthread and libuv port. | ✅ | ❌ | ❌ | ✅ | | libemnapi-basic.a | no atomics feature. no libuv port. napi_*_async_work and napi_*_threadsafe_function are imported from JavaScript land. | ✅ | ✅ | ✅ | ✅ | | libemnapi-basic-mt.a | atomics feature enabled. no libuv port. napi_*_async_work and napi_*_threadsafe_function are imported from JavaScript land. include emnapi_async_worker_create and emnapi_async_worker_init for WebWorker based async work implementation. | ❌ | ✅ | ✅ | ✅ | | libdlmalloc.a | no atomics feature, no thread safe garanteed. | ❌ | ✅ | ❌ | ❌ | | libdlmalloc-mt.a | atomics feature enabled, thread safe. | ❌ | ✅ | ❌ | ❌ | | libemmalloc.a | no atomics feature, no thread safe garanteed. | ❌ | ✅ | ❌ | ❌ | | libemmalloc-mt.a | atomics feature enabled, thread safe. | ❌ | ✅ | ❌ | ❌ |

Usage

add_subdirectory("${CMAKE_CURRENT_SOURCE_DIR}/node_modules/emnapi")

add_executable(hello hello.c)

if(CMAKE_SYSTEM_NAME STREQUAL "Emscripten")
  target_link_libraries(hello emnapi-mt)
  target_compile_options(hello PRIVATE "-pthread")
  target_link_options(hello PRIVATE
    "-sALLOW_MEMORY_GROWTH=1"
    "-sEXPORTED_FUNCTIONS=['_malloc','_free','_napi_register_wasm_v1','_node_api_module_get_api_version_v1']"
    "-pthread"
    "-sPTHREAD_POOL_SIZE=4"
    # try to specify stack size if you experience pthread errors
    "-sSTACK_SIZE=2MB"
    "-sDEFAULT_PTHREAD_STACK_SIZE=2MB"
  )
elseif(CMAKE_C_COMPILER_TARGET STREQUAL "wasm32-wasi-threads")
  target_link_libraries(hello emnapi-mt)
  set_target_properties(hello PROPERTIES SUFFIX ".wasm")
  target_compile_options(hello PRIVATE "-fno-exceptions" "-pthread")
  target_link_options(hello PRIVATE
    "-pthread"
    "-mexec-model=reactor"
    "-Wl,--import-memory"
    "-Wl,--max-memory=2147483648"
    "-Wl,--export-dynamic"
    "-Wl,--export=napi_register_wasm_v1"
    "-Wl,--export-if-defined=node_api_module_get_api_version_v1"
    "-Wl,--export=malloc"
    "-Wl,--export=free"
    "-Wl,--import-undefined"
    "-Wl,--export-table"
  )
elseif((CMAKE_C_COMPILER_TARGET STREQUAL "wasm32") OR (CMAKE_C_COMPILER_TARGET STREQUAL "wasm32-unknown-unknown"))
  target_link_libraries(hello emnapi-basic-mt)
  set_target_properties(hello PROPERTIES SUFFIX ".wasm")
  target_compile_options(hello PRIVATE "-fno-exceptions" "-matomics" "-mbulk-memory")
  target_link_options(hello PRIVATE
    "-nostdlib"
    "-Wl,--no-entry"
    "-Wl,--export=napi_register_wasm_v1"
    "-Wl,--export-if-defined=node_api_module_get_api_version_v1"
    "-Wl,--export=emnapi_async_worker_create"
    "-Wl,--export=emnapi_async_worker_init"
    "-Wl,--import-memory,--shared-memory,--max-memory=2147483648,--import-undefined"
    "-Wl,--export-dynamic,--export=malloc,--export=free,--export-table"
  )
endif()

# emscripten
emcmake cmake -DCMAKE_BUILD_TYPE=Release \
              -DEMNAPI_FIND_NODE_ADDON_API=ON \
              -DEMNAPI_WORKER_POOL_SIZE=4 \
              -G Ninja -H. -Bbuild

# wasi-sdk with thread support
cmake -DCMAKE_TOOLCHAIN_FILE=$WASI_SDK_PATH/share/cmake/wasi-sdk-pthread.cmake \
      -DWASI_SDK_PREFIX=$WASI_SDK_PATH \
      -DEMNAPI_FIND_NODE_ADDON_API=ON \
      -DCMAKE_BUILD_TYPE=Release \
      -G Ninja -H. -Bbuild

cmake -DCMAKE_TOOLCHAIN_FILE=node_modules/emnapi/cmake/wasm32.cmake \
      -DLLVM_PREFIX=$WASI_SDK_PATH \
      -DCMAKE_BUILD_TYPE=Release \
      -G Ninja -H. -Bbuild

cmake --build build

And additional work is required during instantiating wasm compiled with non-emscripten.

// emnapi main thread (could be in a Worker)
instantiateNapiModule(input, {
  context: getDefaultContext(),
  /**
   * emscripten
   *   0: no effect
   *   > 0: the same effect to UV_THREADPOOL_SIZE
   * non-emscripten
   *   0: single thread mock
   *   > 0 schedule async work in web worker
   */
  asyncWorkPoolSize: 4, // 0: single thread mock, > 0: schedule async work in web worker
  wasi: new WASI(/* ... */),

  /**
   * Setting this to `true` or a delay (ms) makes
   * pthread_create() do not return until worker actually start.
   * It will throw error if emnapi runs in browser main thread
   * since browser disallow blocking the main thread (Atomics.wait).
   * @defaultValue false
   */
  waitThreadStart: isNode || (isBrowser && !isBrowserMainThread),

  /**
   * Reuse the thread worker after thread exit to avoid re-creatation
   * @defaultValue false
   */
  reuseWorker: {
    /**
     * @see {@link https://emscripten.org/docs/tools_reference/settings_reference.html#pthread-pool-size | PTHREAD_POOL_SIZE}
     */
    size: 0,

    /**
     * @see {@link https://emscripten.org/docs/tools_reference/settings_reference.html#pthread-pool-size-strict | PTHREAD_POOL_SIZE_STRICT}
     */
    strict: false
  },

  onCreateWorker () {
    return new Worker('./worker.js')
    // Node.js
    // const { Worker } = require('worker_threads')
    // return new Worker(join(__dirname, './worker.js'), {
    //   env: process.env,
    //   execArgv: ['--experimental-wasi-unstable-preview1']
    // })
  },
  overwriteImports (importObject) {
    importObject.env.memory = new WebAssembly.Memory({
      initial: 16777216 / 65536,
      maximum: 2147483648 / 65536,
      shared: true
    })
  }
})

// worker.js
(function () {
  let fs, WASI, emnapiCore

  const ENVIRONMENT_IS_NODE =
    typeof process === 'object' && process !== null &&
    typeof process.versions === 'object' && process.versions !== null &&
    typeof process.versions.node === 'string'

  if (ENVIRONMENT_IS_NODE) {
    const nodeWorkerThreads = require('worker_threads')

    const parentPort = nodeWorkerThreads.parentPort

    parentPort.on('message', (data) => {
      globalThis.onmessage({ data })
    })

    fs = require('fs')

    Object.assign(globalThis, {
      self: globalThis,
      require,
      Worker: nodeWorkerThreads.Worker,
      importScripts: function (f) {
        (0, eval)(fs.readFileSync(f, 'utf8') + '//# sourceURL=' + f)
      },
      postMessage: function (msg) {
        parentPort.postMessage(msg)
      }
    })

    WASI = require('wasi').WASI
    emnapiCore = require('@emnapi/core')
  } else {
    importScripts('./node_modules/memfs-browser/dist/memfs.js')
    importScripts('./node_modules/@tybys/wasm-util/dist/wasm-util.min.js')
    importScripts('./node_modules/@emnapi/core/dist/emnapi-core.js')
    emnapiCore = globalThis.emnapiCore

    const { Volume, createFsFromVolume } = memfs
    fs = createFsFromVolume(Volume.fromJSON({
      '/': null
    }))

    WASI = globalThis.wasmUtil.WASI
  }

  const { instantiateNapiModuleSync, MessageHandler } = emnapiCore

  const handler = new MessageHandler({
    onLoad ({ wasmModule, wasmMemory }) {
      const wasi = new WASI({ fs })

      return instantiateNapiModuleSync(wasmModule, {
        childThread: true,
        wasi,
        overwriteImports (importObject) {
          importObject.env.memory = wasmMemory
        }
      })
    }
  })

  globalThis.onmessage = function (e) {
    handler.handle(e)
    // handle other messages
  }
})()

Preprocess Macro Options

-DEMNAPI_WORKER_POOL_SIZE=4

This is UV_THREADPOOL_SIZE equivalent at compile time, if not predefined, emnapi will read asyncWorkPoolSize option or UV_THREADPOOL_SIZE from Emscripten environment variable at runtime:

Module.init({
  // ...
  asyncWorkPoolSize: 2
})

// if asyncWorkPoolSize is not specified
Module.preRun = Module.preRun || [];
Module.preRun.push(function () {
  if (typeof ENV !== 'undefined') {
    ENV.UV_THREADPOOL_SIZE = '2';
  }
});

// wasi
instantiateNapiModule({
  // ...
  asyncWorkPoolSize: 2
})
// if asyncWorkPoolSize is not specified
new WASI({
  env: {
    UV_THREADPOOL_SIZE: '2'
  }
})

It represent max of EMNAPI_WORKER_POOL_SIZE async work (napi_queue_async_work) can be executed in parallel. Default is not defined.

You can set both PTHREAD_POOL_SIZE and EMNAPI_WORKER_POOL_SIZE to number of CPU cores in general. If you use another library function which may create N child threads in async work, then you need to set PTHREAD_POOL_SIZE to EMNAPI_WORKER_POOL_SIZE * (N + 1).

This option only has effect if you use -pthread. Emnapi will create EMNAPI_WORKER_POOL_SIZE threads when initializing, it will throw error if PTHREAD_POOL_SIZE < EMNAPI_WORKER_POOL_SIZE && PTHREAD_POOL_SIZE_STRICT == 2.

See Issue #8 for more detail.

-DEMNAPI_NEXTTICK_TYPE=0

This option only has effect if you use -pthread, Default is 0. Tell emnapi how to delay async work in uv_async_send / uv__async_close.

• 0: Use setImmediate() (Node.js native setImmediate or browser MessageChannel and port.postMessage)
• 1: Use Promise.resolve().then()

-DEMNAPI_USE_PROXYING=1

This option only has effect if you use emscripten -pthread. Default is 1 if emscripten version >= 3.1.9, else 0.

• 0

  Use JavaScript implementation to send async work from worker threads, runtime code will access the Emscripten internal PThread object to add custom worker message listener.

• 1:

  Use Emscripten proxying API to send async work from worker threads in C. If you experience something wrong, you can switch set this to 0 and feel free to create an issue.