active_storage_fe.js

import { DirectUpload } from 'activestorage'; // https://www.npmjs.com/package/@rails/activestorage

createThing = (name, the_attachment) => { // name is a string, the_attachment is a File object
    const upload = new DirectUpload(
      the_attachment,
      '/rails/active_storage/direct_uploads', // This url is exposed by default in your app
    );
    upload.create((error, blob) => {
      if (error) {
        // Something happened, handle the error
      } else {
        // blob.signed_id, a String, is the key piece of data that lets Rails identify the file we are referring to
        let signed_id = blob.signed_id;
        // BONUS: We can already request the uploaded file from Rails by using this url.
        let url = `/rails/active_storage/blobs/${signed_id}/${"whatever_we_want_the_filename_to_be"}`;
        let request_body = {
          name: name
          the_attachment: signed_id
        }
        post_thing_to_rails(request_body) // This can be REST, or GraphQL, doesn't matter
      }
    });
  };