GLTFExporter and GLTFLoader

when using the parse methods of both GLTFExporter and GLTFLoader you need to feed in the appropriate callbacks and arguments…

const exporter = new GLTFExporter();
const loader = new GLTFLoader();

const options = {
  binary: false,
  maxTextureSize: 1024
};

exporter.parse(cube, (exported) => {

      loader.parse(exported, (loaded) => {
        console.log("loaded", loaded.scene);
      },
      function ( loaded ) {
            scene.add(loaded.scene)
      });
},
  function ( error ) {
  console.log( 'An error happened during parsing', error );
  },
  options
);

you’ll see the console.log("loaded", loaded.scene); is never written to the console as the onLoad function is actually an argument…
there’s not really examples that demonstrate the parsing method of GLTFLoader but it shows this roughly in the documentation three.js docs
the GLTFExporter example does however demonstrate the necessary callbacks and arguments for exporting a gltf, also outlined here three.js docs.