A JavaScript runtime in the BEAM

Note 12 March 2026 Originally on X

I've built a new JavaScript runtime that runs inside the BEAM.

Every JS runtime is a GenServer with its own OS thread. No JSON anywhere — JS objects map to BEAM terms natively through a lock-free queue.

What makes it different from running Node/Deno/Bun alongside Elixir:

JS runtimes live in supervision trees. They crash, restart, recover state — standard OTP
fetch() goes through :httpc. WebSocket through :gun. crypto.subtle through :crypto. BroadcastChannel through :pg — works across a cluster
The DOM is lexbor (C library). JS renders into it, Elixir reads it directly — no serialization, no re-parsing
Workers are BEAM processes. They get preemptive scheduling for free
TypeScript toolchain (OXC) and npm client built in — no Node.js on the machine at all

Full control over the JS layer: parse ASTs, bundle imports, transform TypeScript, minify — all from Elixir via OXC NIFs.

Use cases:

SSR with Preact/React into native DOM, Elixir reads the tree
Sandboxed user-defined business rules with memory limits, timeouts, and a controlled API surface
Parallel Workers that compute and broadcast via distributed process groups
Evaluating or bundling TypeScript without any external toolchain
Running npm packages inside the BEAM

Still a research project in early beta. Covered with tests including Web Platform Tests ports, but expect rough edges.