This is an approach I successfully tried with VSCode. You can use IDE's type checking support in javascript files without any typescript at all. JSDoc annotations facilitate this.

This can be done in two ways.

1. "javascript.implicitProjectConfig.checkJs": true in VSCode configuration
2. create a jsconfig.json in the project repository with checkJs as true.

Reference: https://medium.com/@trukrs/type-safe-javascript-with-jsdoc-7a2a63209b76

My recent experience has been similar to @santhosh's – I wrote a longer summary of my approach in a comment to the sibling ticket about Typescript: https://phabricator.wikimedia.org/T282835#7345366.

Here's another good blog post about the JSDoc way of using Typescript: https://austingil.com/typescript-the-easy-way/

I think we should seriously consider adopting this approach in our new library.

Just in case it's not well known, my team has been using this form of type checking in various web extensions and are using https://github.com/wikimedia/typescript-types to document mediawiki core types.