Browser Target

Io runs in the browser as a WebAssembly module. The browser build compiles the full VM into a WASM reactor module that JS loads and drives via exported functions.

Building

make browser        # build browser/io_browser.wasm
make serve          # serve REPL at http://localhost:8000
make check-browser  # run headless Playwright tests

Requires wasi-sdk for cross-compilation and Node.js + Playwright for headless tests.

Architecture

┌──────────────────────────────────────────────┐
│  Browser (JS)                                │
│                                              │
│  io.js                                       │
│  ├── WASI shim (fd_write, clock, etc.)       │
│  ├── DOM handle table (Map<int, Element>)    │
│  ├── DOM bridge functions (dom namespace)    │
│  ├── WASM loader (loadIo / ioEval)           │
│  └── REPL UI wiring                          │
│                                              │
│  ┌────────────────────────────────────────┐  │
│  │  WebAssembly (io_browser.wasm)         │  │
│  │                                        │  │
│  │  io_browser.c — init, eval, I/O        │  │
│  │  io_dom.c    — DOM/Element protos      │  │
│  │  iovm/*      — full Io VM              │  │
│  └────────────────────────────────────────┘  │
└──────────────────────────────────────────────┘

The WASM module is built with -mexec-model=reactor (no main()). JS calls exported functions:

Export	Purpose
`io_init()`	Initialize the Io VM (call once)
`io_get_input_buf()`	Pointer to 64KB input buffer
`io_eval_input()`	Evaluate code written to input buffer
`io_get_output()`	Pointer to output string
`io_get_output_len()`	Length of output

WASI Shim

The browser has no filesystem or OS. io.js provides a minimal WASI shim:

stdout/stderr — captured to a JS string
clock — performance.now() in nanoseconds
filesystem — all path operations return ENOTCAPABLE
proc_exit — throws a JS Error
random — crypto.getRandomValues()

Files

File	Purpose
`browser/io_browser.c`	Reactor entry point: init, eval, output capture
`browser/io_dom.c`	DOM and Element proto implementation
`browser/io_dom.h`	Header for DOM protos
`browser/io.js`	WASI shim, DOM bridge, WASM loader, REPL UI
`browser/index.html`	REPL page
`browser/test.html`	Automated test page
`browser/run_tests.mjs`	Headless Playwright test runner