npm install --save @types/twitch-ext
This package contains type definitions for twitch-ext (https://dev.twitch.tv/docs/extensions/reference/#javascript-helper).
Files were exported from https://github.com/DefinitelyTyped/DefinitelyTyped/tree/master/types/twitch-ext.
These definitions were written by Benedict Etzel, Federico Della Rovere, and Dmitry Demensky.