Adding Builtin Module

Adding a New Built-in Module

Step-by-step guide for adding a new Node.js-compatible built-in module to the skeleton.

Architecture

Built-in modules use a hybrid native+JS pattern:

• Rust native bridge: Low-level operations via #[rquickjs::module], registered as __wasm_rquickjs_builtin/<name>_native
• JS wrapper: High-level Node.js-compatible API surface, imports from the native bridge

Checklist

1. Create the Rust native module

File: crates/wasm-rquickjs/skeleton/src/builtin/<name>.rs

use rquickjs::module::ModuleDef;
use rquickjs::prelude::*;

pub const <NAME>_JS: &str = include_str!("<name>.js");
pub const REEXPORT_JS: &str = r#"export { default } from 'node:<name>'; export * from 'node:<name>';"#;
pub const WIRE_JS: &str = "";  // Only if globals need wiring

#[rquickjs::module]
mod native_module {
    // Export functions here
    #[rquickjs::function]
    pub fn example_function() -> String {
        "hello".to_string()
    }
}

pub use native_module::js_native_module;

2. Create the JS wrapper

File: crates/wasm-rquickjs/skeleton/src/builtin/<name>.js

import { example_function } from '__wasm_rquickjs_builtin/<name>_native';
// NOTE: Always use original Rust snake_case names in imports!

// Implement Node.js-compatible API surface
const result = example_function();

export { result };
export default { result };

3. Add dependencies (if needed)

Edit crates/wasm-rquickjs/skeleton/Cargo.toml_:

• Always use default-features = false for crates that may pull in C/native libraries
• Use pure-Rust backends (e.g., rust_backend feature) for wasm32-wasip2 compatibility

4. Register in mod.rs

File: crates/wasm-rquickjs/skeleton/src/builtin/mod.rs

Four places need updating:

a) Add the module declaration (top of file):

mod <name>;

b) Add to add_module_resolvers — register the internal native path and public names:

.with_module("__wasm_rquickjs_builtin/<name>_native")
.with_module("node:<name>")
.with_module("<name>")

c) Add to module_loader (ModuleLoader) — map native path to Rust module:

.with_module("__wasm_rquickjs_builtin/<name>_native", <name>::js_native_module)

d) Add to module_loader (BuiltinLoader) — map public names to JS constants:

.with_module("node:<name>", <name>::<NAME>_JS)
.with_module("<name>", <name>::REEXPORT_JS)

e) (Optional) Add to wire_builtins — if the module needs to attach globals:

writeln!(result, "{}", <name>::WIRE_JS).unwrap();

5. Update README.md

Add the new module to the supported APIs list in README.md.

Critical Gotchas

#[rquickjs::module(rename = ...)] does NOT affect JS imports

The rename attribute (e.g., rename = "camelCase") only affects Rust-side naming. JavaScript import statements must always use the original Rust snake_case names. Importing the camelCase version causes "Could not find export" errors.

u32 return value truncation

rquickjs may deliver u32 as signed i32 to JavaScript (e.g., 0xFFFFFFFF → -1). In the JS wrapper, apply >>> 0:

const result = native_fn() >>> 0;

CJS default export immutability

Use Object.defineProperty with writable: false, configurable: false on default export objects to emulate Node.js behavior where module constants are immutable.

WASM-compatible dependencies

Always use default-features = false for crates that may pull in C libraries. Use pure-Rust backends to ensure wasm32-wasip2 target compatibility.

No platform-conditional code

The skeleton is always compiled to wasm32-wasip1. Never write conditional code that checks for unix/windows/macOS or any other host platform (e.g., #[cfg(unix)], #[cfg(windows)], #[cfg(target_os = "...")], process.platform === "win32", path.sep === "\\", etc.). Such checks are meaningless in the WASM target and add dead code complexity.

node:http transport rule

Never use a loopback transport for node:http. Every node:http client request MUST go through wasi:http (the native Rust NodeHttpClientRequest). Do NOT add any fallback that bypasses wasi:http by creating direct node:net socket connections for loopback/localhost addresses.

Testing

1. Define an example in examples/runtime/ (JS file + WIT interface pair)
2. Add tests in tests/runtime.rs that use the example
3. Run: cargo test --test runtime <module> -- --nocapture

If also adding node_compat coverage, use the fixing-node-compat-test skill for that workflow.