Turns out you can define types conditionally based on the type check against a different type. Let's try it out on a fetch function that optionally takes a callback and can return either a void or a promise based on that callback. 00:00 Introduction 00:18 What are conditional types? 02:15 Importing libraries 03:15 fetchPokemon with conditionals 08:23 fetchPokemon with overloading 11:14 Outroduction Conditional Types page: https://www.typescriptlang.org/docs/handbook/2/conditional-types.html Code: https://github.com/jherr/no-bs-ts/tree/master/series-1/episode-18 If you liked this video, buy me a coffee! https://www.buymeacoffee.com/bluecollarcoder 👉 What's my theme? Night Wolf [dark blue] 👉 What's that font? MonoLisa 👉 Jack is also on the React Round Up podcast: https://devchat.tv/podcasts/react-round-up/ 👉 Don't forget to subscribe to this channel for more updates: https://bit.ly/2E7drfJ 👉 Discord server signup: https://discord.gg/ddMZFtTDa5 💢 Watch our other videos: 💟 Hacking your Github contribution graph: https://www.youtube.com/watch?v=_ZQSPYDlk3U 💟 More Typescript videos:    • JavaScript to TypeScript: Beginner to...   Thank you for watching this video, click the "SUBSCRIBE" button to stay connected with this channel. #typescript #NoBSTS