Jco provides a fully native JS toolchain for working with WebAssembly Components in JavaScript.

Features include:

• "Transpiling" Wasm Component binaries into ES modules that can run in any JS environment.
• WASI Preview2 support in Node.js & browsers (experimental).
• Component builds of Wasm Tools helpers, available for use as a library or CLI commands for use in native JS environments, as well as optimization helper for Components via Binaryen.
• Run and serve commands like Wasmtime, as JS implementations of the Command and HTTP Proxy worlds.
• "Componentize" command to easily create components written in JavaScript (wrapper of ComponentizeJS).

For creating components in other languages, see the Cargo Component project for Rust and Wit Bindgen for various guest bindgen helpers.

npm install @bytecodealliance/jco

Jco can be used as either a library import or as a CLI via the jco command.

See the example workflow page for a full usage example.

Usage: jco <command> [options]

jco - WebAssembly JS Component Tools
      JS Component Transpilation Bindgen & Wasm Tools for JS

Options:
  -V, --version                         output the version number
  -h, --help                            display help for command

Commands:
  componentize [options] <js-source>    Create a component from a JavaScript module
  transpile [options] <component-path>  Transpile a WebAssembly Component to JS + core Wasm for JavaScript execution
  types [options] <wit-path>            Generate types for the given WIT
  run [options] <command> [args...]     Run a WASI Command component
  serve [options] <server> [args...]    Serve a WASI HTTP component
  opt [options] <component-file>        optimizes a Wasm component, including running wasm-opt Binaryen optimizations
  wit [options] <component-path>        extract the WIT from a WebAssembly Component [wasm-tools component wit]
  print [options] <input>               print the WebAssembly WAT text for a binary file [wasm-tools print]
  metadata-show [options] [module]      extract the producer metadata for a Wasm binary [wasm-tools metadata show]
  metadata-add [options] [module]       add producer metadata for a Wasm binary [wasm-tools metadata add]
  parse [options] <input>               parses the Wasm text format into a binary file [wasm-tools parse]
  new [options] <core-module>           create a WebAssembly component adapted from a component core Wasm [wasm-tools component new]
  embed [options] [core-module]         embed the component typing section into a core Wasm module [wasm-tools component embed]
  help [command]                        display help for command

For help with individual command options, use jco <cmd> --help.

To transpile a component into JS:

jco transpile component.wasm -o out-dir

The resultant file can be imported providing the bindings of the component as if it were imported directly:

app.js

import { fn } from './out-dir/component.js';

fn();

Imports can be remapped using the --map flag, or to provide imports as an argument use the --instantiation option.

Components relying on WASI bindings will contain external WASI imports, which are automatically updated to the @bytecodealliance/preview2-shim package. This package can be installed from npm separately for runtime usage. This shim layer supports both Node.js and browsers.

Options include:

• --name: Give a custom name for the component JS file in out-dir/[name].js
• --minify: Minify the component JS
• --optimize: Runs the internal core Wasm files through Binaryen for optimization. Optimization options can be passed with a -- <binaryen options> flag separator.
• --tla-compat: Instead of relying on top-level-await, requires an $init promise to be imported and awaited first.
• --js: Converts core Wasm files to JavaScript for environments that don't even support core Wasm.
• --base64-cutoff=<number>: Sets the maximum number of bytes for inlining Wasm files into the JS using base64 encoding. Set to zero to disable base64 inlining entirely.
• --no-wasi-shim: Disable the WASI shim mapping to @bytecodealliance/preview2-shim.
• --map: Provide custom mappings for world imports. Supports both wildcard mappings (* similarly as in the package.json "exports" field) as well as # mappings for targetting exported interfaces. For example, the WASI mappings are internally defined with mappings like --map wasi:filesystem/*=@bytecodealliance/preview2-shim/filesystem#* to map import as * filesystem from 'wasi:filesystem/types' to import { types } from '@bytecodealliance/preview2-shim/filesystem.
• --no-nodejs-compat: Disables Node.js compat in the output to load core Wasm with FS methods.
• --instantiation [mode]: Instead of a direct ES module, export an instantiate function which can take the imports as an argument instead of implicit imports. The instantiate function can be async (with --instantiation or --instantiation async), or sync (with --instantiation sync).
• --valid-lifting-optimization: Internal validations are removed assuming that core Wasm binaries are valid components, providing a minor output size saving.
• --tracing: Emit tracing calls for all function entry and exits.
• --no-namespaced-exports: Removes exports of the type test as "test:flavorful/test" which are not compatible with typescript

To directly call into the transpilation in Rust, the bindgen used in Jco is also available on crates.io as js-component-bindgen.

For Wasm components that implement the WASI Command world, a jco run utility is provided to run these applications in Node.js.

jco run cowasy.component.wasm hello

Using the preview2-shim WASI implementation, full access to the underlying system primitives is provided, including filesystem and environment variable permissions.

For HTTP Proxy components, jco serve provides a JS server implementation:

jco serve --port 8080 server.wasm

Wasmtime generally provides the most performant implementation for executing command and proxy worlds to use. These implementations are rather for when JS virtualization is required or the most convenient approach.

Note: jco componentize is considered experimental, and breaking changes may be made without notice.

To componentize a JS file run:

jco componentize app.js --wit wit -n world-name -o component.wasm

Creates a component from a JS module implementing a WIT world definition, via a Spidermonkey engine embedding.

Currently requires an explicit install of the componentize-js engine via npm install @bytecodealliance/componentize-js.

See ComponentizeJS for more details on this process.

Transpile a Component to JS.

Optimize a Component with the Binaryen Wasm-opt project.

Extract the WIT world from a component binary.

Print the WAT for a Component binary.

Extract the producer toolchain metadata for a component and its nested modules.

Parse a compoment WAT to output a Component binary.

"WIT Component" Component creation tool, optionally providing a set of named adapter binaries.

"WIT Component" Component embedding tool, for embedding component types into core binaries, as an advanced use case of component generation.

Add new producer metadata to a component or core Wasm binary.

See the Contributing chapter of the Jco book.

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

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.