AsyncStorage.js

const asyncToken = callback => {
    AsyncStorage.getItem('TOKEN').then(token => {
        axios.defaults.headers = { authorization: token };
        callback();
    });
};

asyncToken(() => {
    axios.get('http://localhost:3000/auth/testAuth').then((response) => {
        alert(response.data);
    }).catch(function (error) {
        alert(error);
    });
});

you can use async and await for better code readability...

async foo() {
    const token = await AsyncStorage.getItem('TOKEN');
    axios.defaults.headers = { authorization: token };

    // Your HTTP request goes here
}