react-native fetch() cookie persist

fetch on React Native is implemented on top of native-level APIs and differs slightly from the whatwg specification and the well-known github polyfill. This means that when the actual HTTP request is made, it's made by the native networking stack on iOS or OkHttp3 on Android, and in each case, it's the underlying ObjC or Java code that handles and stores the cookies, away from your JS code.

Until this commit of Nov 2015, cookies weren't properly persisted on Android at all, but since RN 0.16 they've been supported on both platforms regardless of the credential setting in your fetch calls. As a result, session cookies and the like work out-of-the-box, which can be disconcerting if you're not expecting anything to be persisted. If you need to manipulate cookies from your JS, have a look at react-native-cookies, which interacts with the underlying cookie stores.