Sveltekit Unexpected token error while running 'npm run build' using @sveltejs/adapter-node

Question

When I try to build my project for production using npm run build I get this error

> Using @sveltejs/adapter-node
error during build:
RollupError: Unexpected token (Note that you need plugins to import files that are not JavaScript)
    at error (file:///Users/marc/Sites/appname.com/node_modules/rollup/dist/es/shared/node-entry.js:2095:30)
    at Module.error (file:///Users/marc/Sites/appname.com/node_modules/rollup/dist/es/shared/node-entry.js:13143:16)
    at Module.tryParse (file:///Users/marc/Sites/appname.com/node_modules/rollup/dist/es/shared/node-entry.js:13820:25)
    at Module.setSource (file:///Users/marc/Sites/appname.com/node_modules/rollup/dist/es/shared/node-entry.js:13430:39)
    at ModuleLoader.addModuleSource (file:///Users/marc/Sites/appname.com/node_modules/rollup/dist/es/shared/node-entry.js:23395:20)

But looking at those sources I don't see any hint of the actual error. Building for dev or preview works perfectly. I didn't select TypeScript when creating my project using npm create svelte@latest appname.com. I updated manually several dependencies in my package.json file since I started this project a long time ago:

  "name": "appname.com",
  "version": "0.0.1",
  "private": true,
  "scripts": {
    "dev": "vite dev",
    "build": "vite build",
    "preview": "vite preview",
    "test": "playwright test",
    "format": "prettier --write ."
  },
  "devDependencies": {
    "@fullcalendar/common": "^5.11.3",
    "@fullcalendar/core": "^5.11.3",
    "@fullcalendar/daygrid": "^5.11.3",
    "@fullcalendar/interaction": "^5.11.3",
    "@playwright/test": "^1.25.0",
    "@sveltejs/adapter-auto": "^2.0.0",
    "@sveltejs/kit": "^1.5.0",
    "@vitejs/plugin-basic-ssl": "^1.0.1",
    "@zerodevx/svelte-toast": "^0.7.2",
    "sass": "^1.53.0",
    "svelte": "^3.54.0",
    "svelte-fullcalendar": "^2.0.0-beta.0",
    "vite": "^4.0.0"
  },
  "type": "module",
  "dependencies": {
    "@sveltejs/adapter-node": "^1.2.1",
    "crypto-js": "^4.1.1",
    "js-cookie": "^3.0.1",
    "runner": "^4.0.0",
    "socket.io-client": "^4.5.2",
    "svelte-local-storage-store": "^0.3.1",
    "svelte-preprocess": "^5.0.1",
    "svelte-scrollto-element": "^0.7.0",
    "webrtc-adapter": "^8.2.0"
  }
}

Answer 1

The node adapter page at https://kit.svelte.dev/docs/adapter-node shows this code to be used in svelte.config.js:

import adapter from '@sveltejs/adapter-node';

export default {
    kit: {
        adapter: adapter()
    }
};

However the original version of svelte.config.js contains this line:

preprocess: vitePreprocess(),

When I left that original line (and its corresponding import), the problem disappeared.