Introduction: Run your JavaScript on WebAssembly. Javy takes your JavaScript code, and executes it in a WebAssembly embedded JavaScript runtime. Javy can create very small Wasm modules in the 1 to 16 KB range with use of dynamic linking. The default static linking produces modules that are at least 869 KB in size.
We welcome feedback, bug reports and bug fixes. We're also happy to discuss feature development but please discuss the features in an issue before contributing.
We will only add JS APIs or accept contributions that add JS APIs that are potentially useful across multiple environments and do not invoke non-WASI hostcalls. If you wish to add or use JS APIs that do not meet these criteria, please use the quickjs-wasm-rs crate directly. We may revisit how we support importing and exporting custom functionality from Javy once the Component Model has stabilized.
- rustup
- Stable Rust (
rustup install stable && rustup default stable) - wasm32-wasi, can be installed via
rustup target add wasm32-wasi - cmake, depending on your operating system and architecture, it might not be
installed by default. On Mac it can be installed with
homebrewviabrew install cmake - Rosetta 2 if running MacOS on Apple Silicon, can be installed via
softwareupdate --install-rosetta - Install the
wasi-sdkby runningmake download-wasi-sdk
- wasmtime-cli, can be installed via
cargo install wasmtime-cli(required forcargo-wasi) - cargo-wasi, can be installed via
cargo install cargo-wasi - wizer, can be installed via
cargo install wizer --all-features
After all the dependencies are installed, run make. You
should now have access to the executable in target/release/javy
Alternatively you can run make && cargo install --path crates/cli.
After running the previous command you'll have a global installation of the
executable.
Define your JavaScript like:
// Read input from stdin
const input = readInput();
// Call the function with the input
const result = foo(input);
// Write the result to stdout
writeOutput(result);
// The main function.
function foo(input) {
return { foo: input.n + 1, newBar: input.bar + "!" };
}
// Read input from stdin
function readInput() {
const chunkSize = 1024;
const inputChunks = [];
let totalBytes = 0;
// Read all the available bytes
while (1) {
const buffer = new Uint8Array(chunkSize);
// Stdin file descriptor
const fd = 0;
const bytesRead = Javy.IO.readSync(fd, buffer);
totalBytes += bytesRead;
if (bytesRead === 0) {
break;
}
inputChunks.push(buffer.subarray(0, bytesRead));
}
// Assemble input into a single Uint8Array
const { finalBuffer } = inputChunks.reduce((context, chunk) => {
context.finalBuffer.set(chunk, context.bufferOffset);
context.bufferOffset += chunk.length;
return context;
}, { bufferOffset: 0, finalBuffer: new Uint8Array(totalBytes) });
return JSON.parse(new TextDecoder().decode(finalBuffer));
}
// Write output to stdout
function writeOutput(output) {
const encodedOutput = new TextEncoder().encode(JSON.stringify(output));
const buffer = new Uint8Array(encodedOutput);
// Stdout file descriptor
const fd = 1;
Javy.IO.writeSync(fd, buffer);
}Create a WebAssembly binary from your JavaScript by:
javy compile index.js -o destination/index.wasmFor more information on the commands you can run javy --help
You can then execute your WebAssembly binary using a WebAssembly engine:
$ echo '{ "n": 2, "bar": "baz" }' | wasmtime index.wasm
{"foo":3,"newBar":"baz!"}% Javy-generated modules are by design WASI only and follow the command pattern. Any input must be passed via stdin and any output will be placed in stdout. This is especially important when invoking Javy modules from a custom embedding.
In a runtime like Wasmtime, wasmtime-wasi can be used to set the input and retrieve the output.
An important use for Javy is for when you may want or need to generate much smaller Wasm modules. Using the -d flag when invoking Javy will create a dynamically linked module which will have a much smaller file size than a statically linked module. Statically linked modules embed the JS engine inside the module while dynamically linked modules rely on Wasm imports to provide the JS engine. Dynamically linked modules have special requirements that statically linked modules do not and will not execute in WebAssembly runtimes that do not meet these requirements.
To successfully instantiate and run a dynamically linked Javy module, the execution environment must provide a javy_quickjs_provider_v1 namespace for importing that links to the exports provided by the javy_quickjs_provider.wasm module. Dynamically linked modules cannot be instantiated in environments that do not provide this import.
Dynamically linked Javy modules are tied to QuickJS since they use QuickJS's bytecode representation.
The javy_quickjs_provider.wasm module is available as an asset on the Javy release you are using. It can also be obtained by running javy emit-provider -o <path> to write the module into <path>.
$ echo 'console.log("hello world!");' > my_code.js
$ javy compile -d -o my_code.wasm my_code.js
$ javy emit-provider -o provider.wasm
$ wasmtime run --preload javy_quickjs_provider_v1=provider.wasm my_code.wasm
hello world!
The quickjs-wasm-rs crate that is part of this project can be used as part of a Rust crate targeting Wasm to customize how that Rust crate interacts with QuickJS. This may be useful when trying to use JavaScript inside a Wasm module and Javy does not fit your needs as quickjs-wasm-rs contains serializers that make it easier to send structured data (for example, strings or objects) between host code and Wasm code.
- Create a tag for the new version like
v0.2.0
git tag v0.2.0
git push origin --tags
- Create a new release from the new tag in github here.
- A GitHub Action will trigger for
publish.ymlwhen a release is published (i.e. it doesn't run on drafts), creating the artifacts for downloading.