How can I get TypeScript-aware autocomplete working with JavaScript files and a `@types` package?

In looking to use a new fairly large project (Three.js) and for teaching beginners, I can see the appeal of auto-complete.

However, for Atom, even with the atom-typescript package, I'm not finding very clear guidance on how to set this up (I have some familiarity with TypeScript syntax but am not used to setting it up myself.). I'd expect for something as useful as type-aware autocomplete for JavaScript, there might be some quick-start tutorials out there for just this use case, but I have not found anything which has helped get things working.

According to https://www.typescriptlang.org/docs/handbook/declaration-files/consumption.html , I'd expect I should just be able to run:

pnpm i -D @types/three

...(using pnpm as my package manager) and then add the import (if the THREE global isn't defined by the @types/three package):

import * as THREE from 'three';

I also ran pnpm i -D typescript figuring Atom might want to access a local copy.

Adding a jsconfig.json with the following (in the root directory with my other files and package.json) did not help:

{
  "compilerOptions": {
    "lib": ["es2015", "dom"]
  }
}

And in atom-typescript I have enabled "Enable Atom-TypeScript for JavaScript files (experimental)". The only other package with "typescript" in the name that I have is "language-typescript" (and I've disabled "ide-typescript").

Even if I need to set up my own declaration file, shouldn't I be getting errors?

Adding a declaration file of my own didn't seem to change anything though. threed.d.ts:

declare module "threed" {
}

I'd expect at least errors about not having types, but not getting anything.

And I also added the following to my ~/.atom/init.coffee file as per https://github.com/TypeStrong/atom-typescript/blob/master/docs/faq.md#i-want-to-use-atom-typescript-with-javascript-too :

#CHANGE THE PACKAGE NAME IN THE NEXT LINE IF YOU'RE USING
#A DIFFERENT GRAMMAR PACKAGE
do (grammarPackageImUsing = "language-javascript") ->
  atom.packages.onDidTriggerActivationHook "#{grammarPackageImUsing}:grammar-used", ->
    atom.packages.triggerActivationHook 'language-typescript:grammar-used'

...and did a restart.

I also tried command-shift-P and TypeScript: Activate. Nothing. What am I missing?