Here is the implementation code to add this feature. This assumes you are compiling a C/C++ based emulator (like Mupen64Plus) to WASM using Emscripten.
// Assuming 'Module' is the Emscripten runtime object const Module = window.Module; n64 wasm
Modern WASM implementations often allow players to sync their save data (SRAM or State files) to the cloud or local storage effortlessly. How to Get Started with N64 WASM Here is the implementation code to add this feature