@rust-js/runtime

The helpers the JS rust-js writes imports: bounds checks, Rust's integer arithmetic and formatting, iterators, JSON for serde, and the rest. Released with the compiler, at its version, as ReScript's @rescript/runtime is. See ADR 0103.

// frontend/src/form.js, as rust-js writes it
import { $debugStr, $parseInt } from "@rust-js/runtime";

A module imports the helpers its code names; the package has the ones they use in turn. Each is a named export, so a bundler keeps only what's used. An app installs the compiler's own version: the build adapter refuses another.

index.js is written by the compiler, from the helpers it has:

bun run runtime        # ./target/debug/rust-js --runtime-module > runtime/index.js