How WebAssembly GC Improves JavaScript Heap Integration

The WebAssembly Garbage Collection (WasmGC) proposal significantly enhances interoperability between WebAssembly and the JavaScript runtime by allowing WebAssembly to directly create, access, and manage garbage-collected data structures. By integrating with the host engine’s native garbage collector rather than forcing languages to compile and run their own internal GC on a flat linear memory buffer, WasmGC eliminates serialization overhead, reduces binary sizes, and ensures seamless reference sharing between JavaScript and WebAssembly.

Direct Object References Without Marshalling

Traditionally, WebAssembly code could only interact with raw byte arrays in a contiguous block of linear memory. Passing complex structures, such as objects or strings, between JavaScript and WebAssembly required serializing data, writing it into linear memory, reading it on the other side, and manually managing its lifecycle. WasmGC introduces first-class GC types—such as structs, arrays, and typed references—allowing WebAssembly to reference JavaScript objects and vice versa directly without copying or serialization bottlenecks.

Unified Cycle Collection and Memory Safety

Prior to WasmGC, keeping cross-boundary references between JavaScript and WebAssembly often led to memory leaks. If a JavaScript object held a handle to linear memory and the internal Wasm memory held a reference back to the JavaScript object, the engine could not naturally trace these circular dependencies. With WasmGC, both JavaScript objects and WebAssembly GC objects are tracked by the same underlying host garbage collector. The engine can trace, mark, and collect circular references across boundaries cleanly and safely.

Smaller Binary Footprints for Managed Languages

Targeting languages with built-in garbage collection (such as Dart, Kotlin, Java, or C#) to standard WebAssembly required compiling an entire GC implementation into the .wasm binary. This inflated download sizes and increased startup latency. By delegating memory allocation and collection to the JavaScript engine’s built-in GC, compiled Wasm binaries are drastically smaller, parse faster, and startup almost instantly.

Seamless Type Sharing

WasmGC allows type representations to bridge the runtime divide. WebAssembly GC structs and arrays can be exposed directly to JavaScript as native-like objects, while JavaScript objects can be received by WebAssembly as typed references (externref or specific GC subtypes). This shared type system allows high-performance compute logic in WebAssembly to read and manipulate the Document Object Model (DOM) and standard web APIs with minimal friction.