react-navigation/example/src/AsyncStorage.tsx

export default {
  getItem(key: string) {
    return Promise.resolve(localStorage.getItem(key));
  },
  setItem(key: string, value: string) {
    return Promise.resolve(localStorage.setItem(key, value));
  },
  removeItem(key: string) {
    return Promise.resolve(localStorage.removeItem(key));
  },
  clear() {
    return Promise.resolve(localStorage.clear());
  },
};