Fetch cancellation in React Native 0.7x: AbortController rejects promptly but native socket teardown is unclear

Answer first

For React Native 0.7x with Hermes, a manual setTimeout that calls controller.abort() is functionally equivalent to AbortSignal.timeout for fetch deadlines. AbortSignal.timeout is not present in RN 0.70-0.71 Hermes builds, so the polyfill is required. Once abort() is invoked, the native request is cancelled via NSURLSessionTask.cancel() on iOS and OkHttp Call.cancel() on Android. The JS promise rejects with a DOMException whose name is 'AbortError' on both platforms, and checking err.name === 'AbortError' is the reliable way to distinguish cancellation from network failures.

Steps for this case

• Detect timeout support at runtime: typeof AbortSignal.timeout === 'function'. If false, use the setTimeout polyfill.
• Wrap fetch with an AbortController and a deadline timer. Clear the timer on success.
• Classify errors by name only: if (e.name === 'AbortError') { /* do not log or retry */ }
• Optionally cancel the response body reader after abort to ensure ReadableStream cancellation propagates to native: response.body?.cancel().

Confirmed facts vs likely explanation

Confirmed

• React Native fetch is a polyfill over platform networking. AbortController.signal is wired to native cancellation APIs.
• AbortSignal.timeout exists in Hermes from ~0.12 onwards, which maps to React Native 0.72+. In RN 0.70-0.71 it is absent.
• Abort rejection is a DOMException with name 'AbortError' across Hermes versions that support AbortController.

Likely explanation, with uncertainty

• Native socket teardown is prompt but not synchronous with the JS microtask rejection. The rejection is scheduled on the JS thread; cancel() runs on the networking thread. A final progress callback can fire once from in-flight buffers, but no new network activity should occur.
• iOS NSURLSession may complete a request that was already fully received before cancel() executes; the promise still rejects with AbortError, which is spec-compliant.
• Android OkHttp marks the call cancelled and closes the socket. If you control the native client, retryOnConnectionFailure can cause a new call to be scheduled by interceptors; abort() cancels the current call only.

Assumption: you are using the built-in React Native fetch. If a custom native fetch bridge is used, AbortController wiring may be incomplete and the above guarantees do not hold.

One diagnostic detail that changes the recommendation: are you using the default RN fetch or a third-party native fetch implementation such as react-native-http-bridge? The native cancel path differs for custom bridges.