How to detect when you are ready to use functions loaded in asm.js module

I'm using emscripten to generate a file a.wasm.js that contains functions in C++ for encryption. The webassembly functions take in and return C-strings that hold JSON data. I want to wrap these asm.js functions for more convenient use in my client application. Unfortunately doing it directly inside body.onload doesn't seem to work so I'm using the following hack:

        <script>
            !function(e, t){
                console.log("Loading client...");
                var n = "a.wasm.js";
                if(!e.WebAssembly){
                    n = "a.js"
                }
                console.log("Script set to " + n);
                var o = t.createElement("script");
                o.async = true, o.type = "text/javascript", o.src = n, o.onerror = function(t) {
                    console.error("Script Error"), console.error(t), setTimeout(function() {
                        e.location.reload(!0)
                    }, 3e3)
                    
                };
                var r = t.getElementsByTagName("script")[0];
                r.parentNode.insertBefore(o, r)
            }(window, document);
        </script>
        <script>
            setTimeout(function(){
                let _generateKeyPair = Module.cwrap('generateKeyPair', 'string', null);
                window.generateKeyPair = function() {
                    return JSON.parse(_generateKeyPair());
                }
            }, 1000)
        </script>

Of course, the problem here is that I cannot use "generateKeyPair" until 1 second after the app is loaded.

Is there any callback I can hook in to to know that the asm.js Module object has fully loaded and I can start to use the functions defined within it?

One approach is to define and await a Module.ready Promise. This is injected during Emscripten compilation using the --post-js flag. (see full examples)

<script src="wasm-module.js"></script>
<script>
  Module.ready
    .then(api => console.log( api.sayHello() ))
    .catch(e => console.error('💩', e))
</script>

$ emcc \
  -O0 `# leave uncompressed for example` \
  -s WASM=1 \
  -s EXPORTED_FUNCTIONS="['_hello']" \
  -s EXTRA_EXPORTED_RUNTIME_METHODS="['cwrap']" \
  -o wasm-module.js \
  --post-js module-post.js \
  hello.c

// hello.c

char *hello() {
  return "Hello there. Welcome to WebAssembly.";
}

// module-post.js

// Module.ready resolves when WASM instantiates. (ready is now a property and not function via @surma fork)
Module.ready = new Promise(function(resolve, reject) {
  addOnPreMain(function() {
    var api = {
      sayHello: Module.cwrap('hello', 'string', [])
    };
    resolve(api);
  });

  // Propagate error to Module.ready.catch()
  // WARNING: this is a hack based Emscripten's current abort() implementation
  // and could break in the future.
  // Rewrite existing abort(what) function to reject Promise before it executes.
  var origAbort = this.abort;
  this.abort = function(what) {
    reject(Error(what));
    origAbort.call(this, what);
  }
});