I was struggling to get React and PropType autocompletion working in my WebStorm installation when working on a
codebase that consists of multiple top-level folders:
/
admin/
index.js
index.html
frontend/ <--- React application resides here.
index.js
index.html
server/
index.php
I went through a number of different solutions trying to get autocompletion working (including installing the
@types/react and @types/prop-types NPM packages).
However, I found that there was only one solution that ended up working:

The solution is as simple as not excluding the specific packages in your node_modules directory (WebStorm excludes
the node_modules directory by default - which means nothing in these directories are indexed).
Once I included node_modules/prop-types and node_modules/react, autocompetion starting working as I expected it to.