| @bytecodealliance/preview2-shim/helpers/filesystem | Host-side filesystem helpers for both builds (see Helpers) |

Preview 2 shim

@bytecodealliance/preview2-shim implements WASI 0.2 (wasi:cli, wasi:clocks, wasi:filesystem, wasi:http, wasi:io, wasi:random and wasi:sockets) for Node.js and browsers. jco transpile maps 0.2.x WASI imports to it by default, so most applications only need it installed:

npm install @bytecodealliance/preview2-shim

Every subpath resolves to the Node.js build under the node export condition and to the browser build otherwise, unless noted:

Subpath	Contents
`@bytecodealliance/preview2-shim`	Every subsystem namespace below, re-exported
`@bytecodealliance/preview2-shim/cli`	`wasi:cli` environment, exit, stdio, and terminal interfaces
`@bytecodealliance/preview2-shim/clocks`	`wasi:clocks` wall and monotonic clocks
`@bytecodealliance/preview2-shim/filesystem`	`wasi:filesystem` types and preopens, plus browser adapters
`@bytecodealliance/preview2-shim/http`	`wasi:http` types and outgoing and incoming handlers
`@bytecodealliance/preview2-shim/io`	`wasi:io` errors, streams, and poll
`@bytecodealliance/preview2-shim/random`	`wasi:random` secure and insecure randomness
`@bytecodealliance/preview2-shim/sockets`	`wasi:sockets` TCP, UDP, and name lookup
`@bytecodealliance/preview2-shim/instantiation`	`WASIShim`, which assembles an import object (see below)
`@bytecodealliance/preview2-shim/interfaces/*`	TypeScript declarations for each WASI interface
`@bytecodealliance/preview2-shim/io-worker`	Node.js only: hooks for extensions running inside the I/O worker
`@bytecodealliance/preview2-shim/browser/helpers/cli`	Browser only: host-side CLI helpers (see Helpers)

Instantiation and sandboxing

WASIShim from @bytecodealliance/preview2-shim/instantiation builds a complete import object for a transpiled component and lets each instance carry its own environment, arguments, stdio handlers, and preopens:

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

const shim = new WASIShim({
    sandbox: {
        preopens: { "/data": "/tmp/guest-data" }, // Node.js: guest path to host path
        env: { MODE: "sandboxed" },
        enableNetwork: false,
    },
});
const component = await instantiate(loader, shim.getImportObject());

See Manual Wasm instantiation with WASI overrides for the full instantiation flow and for replacing individual interfaces.

Helpers

Alongside the WASI implementations, the package exports host-side helper functions from helpers/ subpaths. They build only on the public shim surface and are kept separate from the underscore-prefixed functions (_setStdin, _setPreopens, …) that configure the shim itself. Helpers that only exist for one build are exported under that build’s prefix.

CLI helpers: feed a guest a fixed stdin buffer in the browser.
Filesystem helpers: write and read files under a preopen from host code, on both builds.

Supported platforms

Node.js

The Node.js build gives a component host filesystem, environment, and network access by default, matching the behaviour of ordinary Node.js libraries. It is tested against the Wasmtime WASI conformance suite.

Browsers

The browser build is capability-safe: clocks and randomness use Web APIs, stdout and stderr write to the console, stdin is closed, outbound HTTP delegates to fetch, filesystem preopens are empty until configured, and raw sockets are unavailable unless the application supplies an adapter. Deterministic in-memory filesystem, HTTP, and socket implementations can be opted into for tests and self-contained applications.

The package README carries the full browser support matrix, listing for each interface what is implemented, what needs a host adapter, and what browsers cannot provide.