emsdk/upstream/emscripten/src/postamble_modularize.js

// In MODULARIZE mode we wrap the generated code in a factory function
// and return either the Module itself, or a promise of the module.
//
// We assign to the `moduleRtn` global here and configure closure to see
// this as an extern so it won't get minified.

if (runtimeInitialized)  {
  moduleRtn = Module;
} else {
  // Set up the promise that indicates the Module is initialized
  moduleRtn = new Promise((resolve, reject) => {
    readyPromiseResolve = resolve;
    readyPromiseReject = reject;
  });
}

#if ASSERTIONS
// Assertion for attempting to access module properties on the incoming
// moduleArg.  In the past we used this object as the prototype of the module
// and assigned properties to it, but now we return a distinct object.  This
// keeps the instance private until it is ready (i.e the promise has been
// resolved).
for (const prop of Object.keys(Module)) {
  if (!(prop in moduleArg)) {
    Object.defineProperty(moduleArg, prop, {
      configurable: true,
      get() {
        abort(`Access to module property ('${prop}') is no longer possible via the module constructor argument; Instead, use the result of the module constructor.`)
      }
    });
  }
}
#endif