JavaScript classes implemented in Rust

A class is a b.class({..}) exported from a *.bind.ts file and a Rust struct. src/codegen/bindgen.ts generates the C++ cell, prototype and constructor, the argument conversions, and the Rust functions that call the struct's methods. docs/project/bindgen.mdx is the reference: read its "Classes" section before writing one.

Schema

import * as b from "../../codegen/bindgen-lib.ts";

export const Matcher = b.class({
  rustPath: "crate::api::matcher::Matcher",
  construct: { args: { pattern: b.string.utf8 } },
  internalFields: ["callback"],
  proto: {
    match: { args: { text: b.string.utf8 }, returns: b.bool },
    pattern: { getter: b.string.utf8, cache: true },
  },
});

Rust

pub struct Matcher {
    pattern: Box<[u8]>,
}

impl Matcher {
    pub fn constructor(pattern: &[u8]) -> Self {
        Self { pattern: pattern.into() }
    }

    pub fn r#match(&self, text: &[u8]) -> bool {
        bun_core::strings::contains(text, &self.pattern)
    }

    pub fn get_pattern(&self) -> &[u8] {
        &self.pattern
    }
}

A missing or mis-typed method is a compile error: the generated code calls it directly.

Where the struct lives

• Freed when its JS object is freed: it is stored inside the object. This is the default. Cleanup is Drop.
• Can outlive its object (a timer, another thread or a C library still points at it): refCounted: true. It is a separate, refcounted heap allocation, and the object holds one ref.

What a body may do

capability: "heap" adds heap: &JsHeap<'a> (allocate, throw). capability: "js" adds cx: &mut JsScope<'a> (also run JavaScript); cx.global() is the &JSGlobalObject the rest of the tree takes. throws: true makes the result a JsResult<T>. passThis: true adds the object, this_value: JsWrapper<'a, Self>.

Every member takes &self; state that changes is in Cell or JsCell.

Holding JS values

Never store a raw JSValue in a struct field. Declare an internal field (internalFields: ["callback"] or a cache: true getter) and use js_Matcher::callback_set_cached(this_value, global, value) / callback_get_cached(this_value) from crate::generated_bindgen::<file>. To reach its own object the struct stores a JsRef.

Reference implementations

• src/jsc/bindgen_test.bind.ts + src/jsc/bindgen_test.rs: an example of most keys
• src/jsc/resolve_message.bind.ts: cached getters, prototypeBase, construct: "illegal"
• src/runtime/api/cron.bind.ts: refCounted, finalize