Limitations

wcvm vendors Node's own JavaScript but replaces everything native, so some things differ or are missing. The roadmap and the full list of deliberate differences are in PLAN.md.

What works

fs (sync, callback, promises, streams, watch, glob), stream, events, buffer, util, timers, readline, url, tty, perf_hooks, path, assert, querystring, module, os, child_process (spawn, exec, execFile, execSync, spawnSync, fork with IPC), net (real TCP over a virtual network), http, dgram (UDP), zlib, crypto (hashing), worker_threads, CommonJS, ESM, require(esm), import.meta, and an interactive REPL.

What differs or is missing

• Native add-ons. .node files cannot load. WebAssembly builds work (esbuild-wasm, SWC, Rolldown's browser build).
• crypto is hashing, randomBytes and randomUUID only.
• zlib has no Brotli or Zstd.
• DNS is a fixed-address shim; there is no real name resolution.
• No real network. Sandbox net and http talk to other sandbox processes. Only the built-in npm and wc.fs.fetch reach the internet.
• npm is minimal. No lockfile, no lifecycle scripts, no workspaces, no real npm, pnpm or yarn.
• AsyncLocalStorage is a plain-JavaScript implementation that keeps one current value per instance, so concurrent requests that rely on it share state.
• vm contexts are emulated: no instanceof isolation across contexts.
• V8 internals with no JavaScript equivalent (Promise state, Proxy details, Map and Set iterator previews) are approximated.
• Signals. There is no SIGINT; kill() is SIGTERM or SIGKILL.
• detached children are accepted but not honored: a child always dies with its parent.

Known runtime gaps

• Worker deadlocks with WASI. A napi-rs WebAssembly package that spawns a WASI worker whose file reads relay back to the thread that is blocked waiting on it can deadlock. This is why Nuxt (oxc-parser) does not run, and why Tailwind v4's scanner and Angular's parser needed workarounds.
• child_process.fork worker pools may hang; set JOBS=1 where a tool supports it.
• Vite 8 needs Rolldown, which has no WebAssembly build here. Stay on Vite 7.

Requirements

• A cross-origin isolated page (SharedArrayBuffer).
• A Content-Security-Policy that allows blob: workers.
• A browser with OPFS support if you use persist.
• Node 24 for developing wcvm itself (the unit tests rely on URLPattern and CloseEvent).