Alert.alert 接收到一个字符串，而您从 fetch 响应中得到的内部是一个 com.facebook.react.bridge.ReadableNativeMap 对象(这是 fetch 的本机实现返回的内容).你可以试试:

Alert.alert receives an string, and what you're getting from the fetch response is internally a com.facebook.react.bridge.ReadableNativeMap object (which is what the native implementation for fetch returns). You can try:

Alert.alert(JSON.stringify(responseJson))

如果你使用的是 iOS，你会得到一个完全不同的错误:

If you were using iOS you'll get a completely different error:

Exception '-[_NSFrozenDictionaryM length]: unrecognized selector  ...