Historically, WebAssembly only supported raw linear memory (untyped byte arrays with i32, i64, f32, f64 scalars). When compiling garbage-collected languages like Dart, Kotlin, Java, or C# to WebAssembly, developers had to bundle an entire custom C/C++ garbage collector (such as Boehm GC or Immix) into the WASM binary. This bloated bundle sizes by 5-15 megabytes, caused high initial parse times, and created a dual-heap barrier between the browser JS engine and WebAssembly. WasmGC introduces native structural types (struct, array, i31ref, eqref) that are managed directly by the host browser engine (V8, JavaScriptCore, SpiderMonkey), shrinking bundle sizes by up to 90% and running GC cycles synchronously with the browser.

i31ref is a specialized WasmGC reference type that stores a 31-bit signed integer directly inside the reference pointer itself using a tagged bit (pointer tagging, identical to V8 Smis - Small Integers). Because an i31ref requires zero heap allocation, zero pointer dereferencing, and zero garbage collection tracing overhead, it provides instantaneous boxed integer and enum representation without consuming any GC memory.

In legacy linear-memory WebAssembly, if a JavaScript DOM element referenced a WASM object and that WASM object held a reference back to the JS DOM element, neither garbage collector could trace across the boundary. The JS GC saw only an opaque pointer number, and the WASM GC could not inspect the JS heap, resulting in permanent memory leaks. Because WasmGC objects reside directly in the host browser's unified garbage collection graph (using anyref and externref), the browser's generational/mark-sweep collector traces across JS closures, DOM nodes, and WASM structs seamlessly, collecting cyclic references with zero leaks.

WasmGC is officially supported by: 1) Dart & Flutter Web: Flutter 3.22+ ships WasmGC as the default compilation target, achieving smooth 60/120 FPS UI frame rates and slashing bundle sizes by 50-70%; 2) Kotlin Multiplatform (Kotlin/Wasm): JetBrains Kotlin 2.0+ compiles Kotlin code and Compose Multiplatform directly to WasmGC with zero runtime GC overhead; 3) Java (TeaVM & J2CL): Enables enterprise Java bytecode execution in the browser with native GC integration.