Error [ERR_UNSUPPORTED_DIR_IMPORT]: Directory import when attempting to start Nodejs App locally

Explicitly, point to your main file (usually index.js). E. g.

import ... from './models'          // ❌
import ... from './models/index.js' // ✅

Alternative way (see the below UPDATE):

Use --experimental-specifier-resolution=node flag. For example:

node --experimental-specifier-resolution=node main.js

See:
https://nodejs.org/api/esm.html#esm_customizing_esm_specifier_resolution_algorithm

UPDATE (Node.js v19+):

Node.js has removed the --experimental-specifier-resolution flag. Its functionality can now be achieved via custom loaders.

https://nodejs.org/en/blog/announcements/v19-release-announce/#custom-esm-resolution-adjustments

The simplest loader (that only appends .js extension if needed) is something like:

import {isBuiltin} from 'node:module'

// noinspection JSUnusedGlobalSymbols
export const resolve = (specifier, context, nextResolve) => // This function can be `async` too
  nextResolve(isBuiltin(specifier) || specifier.endsWith('.js') ? specifier : `${specifier}.js`, context)

Name it loader.js (or loader.mjs if you don’t set yet "type": "module" in your package.json).

Then if you run your script (e.g. ./some-script.js) using this loader:

node --loader ./loader.js some-script.js

the imports within some-script.js (and the files imported in it) can omit .js extension.

See more complex examples: https://github.com/nodejs/loaders-test