A developer built WATaBoy, a Game Boy emulator that compiles SM83 instructions to WebAssembly instead of native machine code. By targeting WASM bytecode, the emulator sidesteps iOS restrictions on JIT compilation while leveraging Safari's JavaScriptCore JIT capabilities. The project emerged from curiosity about running CPU-intensive emulators like Dolphin on platforms that prohibit direct JIT compilation.

Apple's iOS platform blocks JIT compilation for security reasons, preventing emulators like Dolphin from reaching the App Store. However, web browsers receive an exception, allowing JavaScriptCore to JIT-compile JavaScript and WASM code. This creates an opportunity: generate WASM bytecode at runtime, let the browser compile it to native code, then execute. Projects like The Jiterpreter and v86 already use this technique, but WATaBoy appears to be the first gaming console emulator implementation.

The implementation uses Rust with the wasm-encoder crate to generate WASM bytecode at runtime, avoiding higher-level tools like wasm-bindgen that introduce ergonomic overhead. Data passes between Rust and JavaScript via the C ABI using pointers and buffer lengths. The approach requires Nightly Rust due to inline WASM usage, employing indirect function tables for dispatch.

WATaBoy serves primarily as a proof of concept comparing JIT-to-WASM performance against native interpretation. While Game Boy emulation doesn't demand the intensive computation of sixth-generation consoles, the technique demonstrates viable performance gains while maintaining cycle accuracy through interrupt prediction and interpreter fallback mechanisms.