demo · v137

JSPI — WebAssembly suspends on JS Promises

JSPI lets a C / C++ port call a synchronous-looking host function that actually awaits a JS Promise — the Wasm stack suspends. Below: a tiny Wasm module imports an async sleep and treats it like a blocking call. The button runs it; observe how the JS side stays responsive.

Heads upJSPI requires the WebAssembly.promising wrapper. If your build is older, the probe will say so and the button will use a non-JSPI fallback that demonstrates the difference.

probing…

no run yet

the code

// Async import to Wasm:
const imports = {
  env: {
    sleep: (ms) => new Promise((r) => setTimeout(r, ms)),
  },
};
const { instance } = await WebAssembly.instantiate(bytes, imports);

// Wrap the export so it can suspend on Promises:
const runAsync = WebAssembly.promising(instance.exports.run);
await runAsync(3, 1000);  // wasm calls sleep() 3× as if it were synchronous

the code

see also