wasm.loader.js

var importObject = {
  imports: {
    imported_func: function(arg) {
      console.log(arg);
    }
  }
};

fetch('simple.wasm').then(response =>
  response.arrayBuffer()
).then(bytes =>
  WebAssembly.instantiate(bytes, importObject);
).then(function(mod) {
  var imports = WebAssembly.Module.imports(mod);
  console.log(imports[0]);
});