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The StarlingMonkey JS Runtime

StarlingMonkey is a SpiderMonkey based JS runtime optimized for use in WebAssembly Components. StarlingMonkey's core builtins target WASI 0.2.0 to support a Component Model based event loop and standards-compliant implementations of key web builtins, including the fetch API, WHATWG Streams, text encoding, and others. To support tailoring for specific use cases, it's designed to be highly modular, and can be readily extended with custom builtins and host APIs.

Requirements

The runtime's build is managed by cmake, which also takes care of downloading the build dependencies. To properly manage the Rust toolchain, the build script expects rustup to be installed in the system.

Building and Running

With sufficiently new versions of cmake and rustup installed, the build process is as follows:

  1. Clone the repo
git clone https://github.com/bytecodealliance/StarlingMonkey
cd StarlingMonkey
  1. Run the configuration script

For a release configuration, run

cmake -S . -B cmake-build-release -DCMAKE_BUILD_TYPE=Release

For a debug configuration, run

cmake -S . -B cmake-build-debug -DCMAKE_BUILD_TYPE=Debug
  1. Build the runtime

The following command will build the starling.wasm runtime module in the cmake-build-release directory:

# Use cmake-build-debug for the debug build
# Change the value for `--parallel` to match the number of CPU cores in your system
cmake --build cmake-build-release --parallel 8
  1. Testing the build

After completing the build (a debug build in this case), the integration test runner can be built:

cmake --build cmake-build-debug --target integration-test-server

Then tests can be run with ctest directly via:

ctest --test-dir cmake-build-debug -j8 --output-on-failure

Alternatively, the integration test server can be directly run with wasmtime serve via:

wasmtime serve -S common cmake-build-debug/test-server.wasm

Then visit http://0.0.0.0:8080/timers, or any test name and filter of the form [testName]/[filter]

  1. Using the runtime with other JS applications

The build directory contains a shell script componentize.sh that can be used to create components from JS applications. componentize.sh takes a single argument, the path to the JS application, and creates a component with a name of the form [input-file-name].wasm in the current working directory.

For example, the following command is equivalent to the cmake invocation from step 5, and will create the component cmake-build-release/smoke.wasm:

cd cmake-build-release
./componentize.sh ../tests/smoke.js

Thorough testing with the Web Platform Tests suite

StarlingMonkey includes a test runner for the Web Platform Tests suite. The test runner is built as part of the starling.wasm runtime, and can be run using the wpt-test target.

Requirements

The WPT runner requires Node.js to be installed, and during build configuration the environment variable WPT must be defined.

When running the test, WPT_ROOT must be set to the path of a checkout of the WPT suite at revision 1014eae5e66f8f334610d5a1521756f7a2fb769f:

WPT=1 WPT_ROOT=[path to your WPT checkout] cmake -S . -B cmake-build-debug -DCMAKE_BUILD_TYPE=Debug
cmake --build cmake-build-debug --parallel 8 --target wpt-runtime
cd cmake-build-debug
ctest --verbose # Note: some of the tests run fairly slowly in debug builds, so be patient

Configuring available builtins

StarlingMonkey supports enabling/disabling bundled builtins using CMake options. You can get a full list of bundled builtins by running the following shell command:

cmake -P [PATH_TO_STARLING_MONKEY]/cmake/builtins.cmake

Note that it's required to include builtins defining all exports defined by the used host API. Using the default WASI 0.2.0 host API, that means including the fetch_event builtin.

Using StarlingMonkey as a CMake sub-project

StarlingMonkey can be used as a subproject in a larger CMake project.

The importing project must at a minimum contain the following line in its CMakeLists.txt:

include("${PATH_TO_STARLING_MONKEY}/cmake/add_as_subproject.cmake")

With that line added (and ${PATH_TO_STARLING_MONKEY} replaced with the actual path to StarlingMonkey), the importing project will have all the build targets of StarlingMonkey available to it.

Note that building the starling.wasm target itself will result in the linked starling.wasm file being created in the starling.wasm sub-directory of the importing project's build directory.

To make use of importing StarlingMonkey in this way, you'll probably want to add additional builtins, or provide your own implementation of the host interface.

Adding custom builtins

Adding builtins is as simple as calling add_builtin in the importing project's CMakeLists.txt. Say you want to add a builtin defined in the file my-builtin.cpp, like so:

// The extension API is automatically on the include path for builtins.
#include "extension-api.h"

// The namespace name must match the name passed to `add_builtin` in the CMakeLists.txt
namespace my_project::my_builtin {

    bool install(api::Engine* engine) {
        printf("installing my-builtin\n");
        return true;
    }

} // namespace my_builtin

This file can now be included in the runtime's builtins like so:

add_builtin(my_project::my_builtin SRC my-builtin.cpp)

If your builtin requires multiple .cpp files, you can pass all of them to add_builtin as values for the SRC argument.

Providing a custom host API implementation

The host-apis directory contains implementations of the host API for different versions of WASI. Those can be selected by setting the HOST_API environment variable to the name of one of the directories. By default, the wasi-0.2.0 host API is used.

To provide a custom host API implementation, you can set HOST_API to the (absolute) path of a directory containing that implementation.