lamassu-jsA JavaScript subset, sandboxed

A strict, safe JavaScript-subset engine written in C, compiled to WebAssembly — for running untrusted scripts inside a web-framework templating language.

Read the Guide

API Reference

Running untrusted code

Try it live

Built for untrusted input

A per-instruction CPU budget, a hard heap cap, an interrupt safe to call from a signal handler, bounded stack depth, and a bytecode loader that treats its input as hostile. The limits are opt-in — set them, and a guest that never stops is a caught error rather than a hung process.

How to set the limits

Compile once, run repeatedly

Source compiles to a versioned, validated bytecode format. Cache it, ship it, run it — the validating loader treats bytecode as hostile input, never as trusted state.

Real async, two ways

Guest-level await of a native-returned promise for ordinary host calls, and a synchronous-looking __hostcall for embedders who'd rather not thread await through guest code.

Small, deliberate subset

let/const only, strict mode only, no classes/eval/Function constructor. Every omission is a design decision, documented — not a missing feature.