Using Twitch's Dev Rig

By using Twitch’s Dev Rig you can test your extension locally against a mock version of their API’s or you can choose to test in a hosted environment that uses Twitch’s production API’s and hosted assets.

https://github.com/twitchdev/developer-rig

Improved Dev Rig

https://blog.twitch.tv/welcome-to-a-new-more-intuitive-developer-rig-8581a640b00