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0.0.3 • Public • Published

waPC JavaScript Host (Node + Browser)

This is the JavaScript implementation of the waPC standard for WebAssembly host runtimes. It allows any WebAssembly module to be loaded as a guest and receive requests for invocation as well as to make its own function requests of the host.

This package defines the protocol for RPC exchange between guest (WebAssembly) modules and the host runtime.

Who is this for?

Library developers who want to load waPC-compliant WebAssembly binaries in nodejs or browser environments. The waPC protocol allows a host to execute arbitrary wasm functions without knowing the exported bindings beforehand.

While waPC does not prescribe a serialization/deserialization format, MessagePack is common recommended unless you need otherwise.


$ npm install @wapc/host



const { instantiate } = require('@wapc/host');
const { encode, decode } = require('@msgpack/msgpack');
const { promises: fs } = require('fs');
const path = require('path');

async function main() {
  // Read wasm off the local disk as Uint8Array
  buffer = await fs.readFile(path.join(__dirname, 'fixtures', 'rust_echo_string.wasm'));

  // Instantiate a WapcHost with the bytes read off disk
  const host = await instantiate(buffer);

  // Encode the payload with MessagePack
  const payload = encode({ msg: 'hello world' });

  // Invoke the operation in the wasm guest
  const result = await host.invoke('echo', payload);

  // Decode the results using MessagePack
  const decoded = decode(result);

  // log to the console
  console.log(`Result: ${decoded}`);

main().catch(err => console.error(err));


Browser environments can instantiate from a FetchResponse with waPC.instantiateStreaming(). If the browser does not support streaming wasm then waPC gracefully degrades and waits for the response to complete before instantiating.

<script src="https://unpkg.com/@msgpack/msgpack@2.5.1/dist.es5+umd/msgpack.min.js"></script>
<script src="dist/index.bundle.js"></script>
  (async () => {
    // Fetch wasm bytes
    response = await fetch('/test/fixtures/rust_echo_string.wasm');

    // Instantiate a WapcHost with fetch response object
    const host = await waPC.instantiateStreaming(response);

    // Encode the arguments to send to the wasm function (Example using msgpack)
    const payload = MessagePack.encode({ msg: 'hello world' });

    // Invoke the "echo" function in the wasm guest
    const result = await host.invoke('echo', payload);

    // Decode the results (using msgpack in this example)
    const decoded = MessagePack.decode(result);


Running tests

Node unit tests

$ npm run test:unit

Browser unit tests

$ npm run test:browser


$ npm run build




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npm i @wapc/host

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  • pkedy
  • jsoverson
  • wapcbot