webworker.js

 var g_importObject = {
   'env': {
     'memoryBase': 0,
     'tableBase': 0,
     'memory': new WebAssembly.Memory({ initial: 256 }),
     'table': new WebAssembly.Table({ initial: 0, element: 'anyfunc' }),
    'abort': function(){},
    'setTempRet0': function(){},
    'getTempRet0': function(){}
   }
 };
// The WebAssembly module instance that we'll be working with
var g_objInstance = null;
// Listen for messages from the main thread. Because all messages to this thread come through
// this method, we need a way to know what is being asked of us which is why we included the
// MessagePurpose property.
self.onmessage = function (evt) {
  // If we've been asked to call the module's Add method then...
  var objData = evt.data;
  var sMessagePurpose = objData.MessagePurpose;
  if (sMessagePurpose === "AddValues") {
    // Call the add method in the WebAssembly module and pass the result back to the main thread
    var iResult = g_objInstance.exports._mmixal_wasm(objData.Val1);
    self.postMessage(`This is the Web Worker...The result of ${objData.Val1.toString()} `);
  } // If we've been passed a compiled WebAssembly module then...
  else if (sMessagePurpose === "CompiledModule") {
    // NOTE: Unlike when we pass in the bytes to instantiate, we don't have a separate 'instance'
    // and 'modules' object returned in this case since we started out with the module object.
    // We're only passed back the instance in this case.
    // WebAssembly.instantiateStreaming(fetch("mmix.wasm"), g_importObject).then(instance => g_objInstance = instance);
     WebAssembly.instantiate(objData.WasmModule, g_importObject).then(instance => 
       g_objInstance = instance,
         reason => console.debug(reason)// Hold onto the module's instance so that we can reuse it
     );
  }
}