Most popular libraries these days either include their own typescript definitions, or the definitions are available vie @types (it is as easy as npm install --save-dev @types/library-name)

If on the off chance that no typings are available, you just write a simple namespace declaration (usually a one liner) and start using the library right away, albeit without smart code sensing / completion. See https://stackoverflow.com/questions/22842389/how-to-import-a...