bun-src/.claude/skills/implementing-jsc-classes-rust/SKILL.md

name, description

name	description
implementing-jsc-classes-rust	Creates JavaScript classes using Bun's Rust bindings generator (.classes.ts). Use when implementing new JS APIs in Rust with JSC integration, prototypes, or constructors.

Bun's JavaScriptCore Class Bindings Generator

Bridge JavaScript and Rust through .classes.ts definitions and Rust implementations.

Architecture

1. JavaScript Interface Definition (.classes.ts files)
2. Rust Implementation (.rs files)
3. Generated Code — src/codegen/generate-classes.ts emits C++ + Rust into ${BUN_CODEGEN_DIR}/generated_classes.rs, include!d as crate::generated_classes in bun_runtime. Run bun bd to regenerate.

Class Definition (.classes.ts)

export default [
  define({
    name: "Glob",
    construct: true,
    finalize: true,
    hasPendingActivity: true,
    proto: {
      scan:  { fn: "scan",  length: 1 },
      match: { fn: "match", length: 1 },
    },
  }),
];

Options:

• construct: Has a public new Foo() constructor
• finalize: Needs cleanup beyond Drop (rarely — see Finalize below)
• hasPendingActivity: GC keep-alive while async work is in flight
• proto: Methods (fn:), getters (getter: true, optionally cache: true)
• values: [...]: WriteBarrier slots for JS values the native side holds (callbacks, buffers)

Rust Implementation

use bun_jsc::{CallFrame, JSGlobalObject, JSValue, JsResult};
use std::sync::atomic::{AtomicUsize, Ordering};

#[bun_jsc::JsClass]
pub struct Glob {
    pattern: Box<[u8]>,
    has_pending_activity: AtomicUsize,
}

impl Glob {
    pub fn constructor(global: &JSGlobalObject, frame: &CallFrame) -> JsResult<Box<Glob>> {
        let arg = frame.argument(0);
        let pattern = bun_core::String::from_js(arg, global)?.to_utf8_bytes().into();
        Ok(Box::new(Glob { pattern, has_pending_activity: AtomicUsize::new(0) }))
    }

    #[bun_jsc::host_fn(method)]
    pub fn r#match(&self, global: &JSGlobalObject, frame: &CallFrame) -> JsResult<JSValue> {
        // ...
        Ok(JSValue::TRUE)
    }

    pub fn has_pending_activity(&self) -> bool {
        self.has_pending_activity.load(Ordering::SeqCst) > 0
    }
}

Canonical signatures

Hook	Signature
constructor	pub fn constructor(global: &JSGlobalObject, frame: &CallFrame) -> JsResult<Box<Self>>
method (fn:)	pub fn name(&self, global: &JSGlobalObject, frame: &CallFrame) -> JsResult<JSValue>
getter	pub fn get_x(this: &Self, global: &JSGlobalObject) -> JsResult<JSValue>
finalize	pub fn finalize(self: Box<Self>) — or omit; the blanket JsFinalize just drops
hasPendingActivity	pub fn has_pending_activity(&self) -> bool

A missing or mis-typed hook is a compile error in cargo check -p bun_runtime — the generated code calls the inherent method directly.

Hooking into the generated module

#[bun_jsc::JsClass] on the struct implements the JsClass trait (to_js, from_js, from_js_direct, get_constructor) by binding the C++ externs. Attribute knobs: no_constructor, no_finalize, estimated_size.

The codegen also emits a js_$T module with the cached-value accessors. Re-export it when you need *_set_cached / *_get_cached or detach_ptr:

pub use crate::generated_classes::js_Glob as js;
// or
bun_jsc::impl_js_class_via_generated!(Archive => crate::generated_classes::js_Archive);

The js_$T module surface:

pub fn from_js(value: JSValue) -> Option<NonNull<T>>;
pub fn from_js_direct(value: JSValue) -> Option<NonNull<T>>;
pub fn get_constructor(global: &JSGlobalObject) -> JSValue;
pub fn to_js(this: *mut T, global: &JSGlobalObject) -> JSValue;   // ownership transfer
pub fn detach_ptr(value: JSValue);
// per cached getter / `values: [...]` entry:
pub fn <field>_set_cached(this_value: JSValue, global: &JSGlobalObject, value: JSValue);
pub fn <field>_get_cached(this_value: JSValue) -> Option<JSValue>;

Finalize

Most classes need nothing — #[bun_jsc::JsClass] wires the blanket JsFinalize whose default is drop(Box<Self>). Override only when you must release a JS handle or defer to a heap helper:

pub fn finalize(self: Box<Self>) {
    bun_ptr::finalize_js_box(self, |this| this.this_value.with_mut(|v| v.finalize()));
}

Override with an inherent method, never impl JsFinalize for T.

Holding JS values

Never store raw JSValue in a struct field. Declare a slot in .classes.ts (values: ["callback"] or a cache: true getter) and read/write it through js::callback_set_cached(this_value, global, v) / js::callback_get_cached(this_value). The slot is a WriteBarrier visited by the GC, so the value stays alive without a Strong.

Reference implementations

• src/runtime/api/glob.rs + Glob.classes.ts — constructor, methods, hasPendingActivity, default finalize
• src/runtime/api/cron.rs + cron.classes.ts — noConstructor, cached getter, values: [...], custom finalize
• src/runtime/image/Image.rs:56 — the pub use crate::generated_classes::js_Image as js; one-liner
• src/jsc/host_fn.rs — the host-fn adapters the codegen dispatches through
• src/jsc_macros/lib.rs — #[bun_jsc::JsClass] proc-macro source