See live demo and browser demo sources

For most WASI components, instantiateWasiComponent is the simplest way:

import { instantiateWasiComponent } from '@pavelsavara/jsco';

// Works with both WASIp2 and WASIp3 components (auto-detected)
const instance = await instantiateWasiComponent('./my-component.wasm');
await instance.exports['wasi:cli/run@0.3.0'].run();

For advanced use cases (custom imports, non-WASI components), use createComponent directly:

import { createComponent } from '@pavelsavara/jsco';

const component = await createComponent('./my-component.wasm');
const instance = await component.instantiate({
    'my:app/logger@1.0.0': { log: console.log },
});
await instance.exports['my:app/greeter@1.0.0'].run({ name: 'World' });

import { instantiateWasiComponent } from '@pavelsavara/jsco';

// Run a CLI component with real filesystem access
const instance = await instantiateWasiComponent('./my-cli-component.wasm', {
    args: ['input.txt'],
    env: [['HOME', '/home/user']],
    mounts: [{ hostPath: './data', guestPath: '/data' }],
});
await instance.exports['wasi:cli/run@0.3.0'].run();

To serve an HTTP handler component:

import { createComponent, loadWasiP3Host, loadWasiP3Serve } from '@pavelsavara/jsco';

const { createWasiP3Host } = await loadWasiP3Host();
const host = createWasiP3Host();
const component = await createComponent('./my-http-component.wasm');
const instance = await component.instantiate(host);
const handler = instance.exports['wasi:http/incoming-handler@0.2.0'];
const { serve } = await loadWasiP3Serve();
const handle = await serve(handler, { network: { httpRequestTimeoutMs: 30_000 } });
console.log(`Listening on port ${handle.port}`);

createWasiP3Host() accepts an optional HostConfig:

Each WASI interface is implemented for one or more preview generations (P1 = wasi_snapshot_preview1, P2 = wasi:*@0.2.x, P3 = wasi:*@0.3.x), and may behave differently in the browser vs. Node.js.

P1 modules are served by the wasip1-via-wasip3 adapter (wasi_snapshot_preview1 shim layered on top of the P3 host); same configuration, same VFS/mount/network options apply.

jsco follows a command-based CLI similar to wasmtime. If no subcommand is provided, run is used by default.

# Run a component (default command)
jsco run ./integration-tests/hello-p2-world-wat/hello.wasm
# or without the subcommand
jsco ./integration-tests/hello-p2-world-wat/hello.wasm

# Serve an HTTP proxy component
jsco serve --addr 0.0.0.0:8080 ./my-http-component.wasm

# Show help
jsco --help
jsco run --help
jsco serve --help

When installed locally or via npx:

npx @pavelsavara/jsco run ./integration-tests/hello-p2-world-wat/hello.wasm

DOS-mitigation budgets enforced by the runtime. Each can be tuned via CLI flag or programmatically via instantiate(..., { limits: { ... } }).

HTTP/socket budgets enforced by the host. Run jsco run --help for the full list.

• browser polyfill for running WASM/WASI components.
• streaming parser of binary WIT
• streaming compilation of WASM core modules during .wasm file download
• in-the-browser creation of instances and necessary JavaScript interop
• keep download size small enough to be practical for browser use

• to provide host which could do the binding in the browser
• browsers currently don't implement built-in WASM component model host
• because independent implementation will help the WASM/WIT/WASI to make progress
• JCO is great alternative, really.
  • But it is too large to use as dynamic host, because download size matters to browser folks.
  • When you have all your components available at dev machine, JCO transpiler could be better choice.

• parser: read binary WIT to produce model of the component, it's sub components, modules and types
• compile modules via Browser API WebAssembly.compileStreaming
• resolver: resolve dependencies, create instances, bind it together.
• JS binding: for component's imports and exports
• just JS at runtime (no rust dependency)
• TypeScript, RollupJS, rust as dev time dependencies

🚧 Preview quality 🚧

The WASIp1/p2/p3 hosts are implemented and pass an extensive test suite, but the API surface is not yet stable and behavior may change between preview releases. Not recommended for production use yet. Also this is one man + 🤖 show, come and help with testing.

See ./CHANGELOG.md for release notes and ./TODO.md for the roadmap. Contributors are welcome!

See ./jspi.md for more details about JSPI - synchronous calls to JS APIs which are blocking, like I/O.

This project is licensed under the Apache License, Version 2.0, with the LLVM exception. See LICENSE for details, and THIRD-PARTY-NOTICES.TXT for attribution of code adapted from upstream projects.

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in this project by you, as defined in the Apache-2.0 license, shall be licensed as above, without any additional terms or conditions.