This is a JavaScript wrapper intended to be used with Asyncify feature of Binaryen.

Together, they allow to use asynchronous APIs (such as most Web APIs) from within WebAssembly written and compiled from any source language.

Import and use required APIs as regular synchronous FFI functions in your code.

After the code is compiled to WebAssembly, post-process it using wasm-opt from the Binaryen toolchain:

wasm-opt --asyncify [-O] [--pass-arg=asyncify-imports@module1.func1,...] in.wasm -o out.wasm

First, import asyncify via:

import * as Asyncify from 'https://unpkg.com/asyncify-wasm?module';

Compilation / instantiation APIs are designed to be drop-in replacements for those of regular WebAssembly interface, but with async support.

Then, you can use new Asyncify.Instance, Asyncify.instantiate and Asyncify.instantiateStreaming like you would with corresponding WebAssembly functions, but with added support for async imports and all exports wrapped into async functions, too.

For example:

let { instance } = await Asyncify.instantiateStreaming(fetch('./out.wasm'), {
  get_resource_text: async url => {
    let response = await fetch(readWasmString(instance, url));
    if (!response.ok) {
      throw new Error(`HTTP ${response.status}: ${response.statusText}`);
    }
    return passStringToWasm(instance, await response.text());
  }
});

await instance.exports._start();