Preview2 Shim

WASI Preview2 implementations for Node.js & browsers.

Node.js support is fully tested and conformant against the Wasmtime test suite.

Browser support is considered experimental, and not currently suitable for production applications.

Features

WASI Shim object for easy instantiation

An default instantiation object can be used via the WASIShim class in @bytecodealliance/preview2-shim/instantiation:

import { WASIShim } from '@bytecodealliance/preview2-shim/instantiation';
import type {
    VersionedWASIImportObject,
    WASIImportObject,
} from '@bytecodealliance/preview2-shim/instantiation';

const shim = new WASIShim();

const unversioned: WASIImportObject = shim.getImportObject();
// console.log('unversioned', unversioned);
unversioned satisfies WASIImportObject;
unversioned satisfies VersionedWASIImportObject<''>;

const versioned: VersionedWASIImportObject<'0.2.3'> = shim.getImportObject({
    asVersion: '0.2.3',
});
//console.log('versioned', versioned);
versioned satisfies VersionedWASIImportObject<'0.2.3'>;

The import object generated by getImportObject can be easily used in instantiate() calls produced by jco transpile (with --instantiation=async):

import { WASIShim } from '@bytecodealliance/preview2-shim/instantiation';

// The code below assumes that you have output your transpiled WebAssembly component to `dist/transpiled`
import { instantiate } from './dist/transpiled/component.js';

const loader = async (path: string) => {
    const buf = await readFile(`./dist/transpiled/${path}`);
    return await WebAssembly.compile(buf.buffer as ArrayBuffer);
};
const component = await instantiate(loader, new WASIShim().getImportObject());

// TODO: Code that uses your component's exports goes here.

Sandboxing

By default, the preview2-shim provides full access to the host filesystem, environment variables, and network - matching the default behavior of Node.js libraries. However, you can configure sandboxing to restrict what guests can access.

Using WASIShim for sandboxing

The WASIShim class accepts a sandbox configuration option to control access:

import { WASIShim } from '@bytecodealliance/preview2-shim/instantiation';

// Fully sandboxed - no filesystem, network, or env access
const sandboxedShim = new WASIShim({
  sandbox: {
    preopens: {},           // No filesystem access
    env: {},                // No environment variables
    args: ['arg1'],         // Custom arguments
    enableNetwork: false,   // Disable network access
  }
});

// Limited filesystem access - map virtual paths to host paths
const limitedShim = new WASIShim({
  sandbox: {
    preopens: {
      '/data': '/tmp/guest-data',  // Guest sees /data, maps to /tmp/guest-data
      '/config': '/etc/app'        // Guest sees /config, maps to /etc/app
    },
    env: { 'ENV1': '42' },           // Only expose specific env vars
  }
});

const component = await instantiate(loader, sandboxedShim.getImportObject());

Notes on sandboxing

• By default (when no options are passed), the shim is providing full access to match typical Node.js library behavior.
• Each WASIShim instance has its own isolated preopens, environment variables, and arguments. Multiple instances with different configurations will not affect each other.
• The direct preopen functions (_setPreopens, _clearPreopens, etc.) modify global state and affect all components not using WASIShim with explicit configuration. For isolation, prefer using WASIShim with the sandbox option containing preopens and env.
• When sandbox.enableNetwork: false, all socket and HTTP operations will throw "access-denied" errors.

License

This project is licensed under the Apache 2.0 license with the LLVM exception. See LICENSE for more details.

Contribution

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.