How to check if a key exists in AsyncStorage in React Native? getItem() always returns a promise object

You need to add async await, or add .then to the result

async checkUserSignedIn(){
    let context = this;
    try {
       let value = await AsyncStorage.getItem('user');
       if (value != null){
          // do something 
       }
       else {
          // do something else
      }
    } catch (error) {
      // Error retrieving data
    }
}

Like the name says, it's async. So you have to:

AsyncStorage.getItem('user')
.then((item) => {
     if (item) {
       // do the damage
     }
});

If needed, you can play either with local state or some application state management library.

AsyncStorage is async... so, you need to call it this way:

checkUserSignedIn(callback){
  AsyncStorage.getItem('user', (err, result) => {
    if (!err && result != null){
        // do something 
    }
    else {
        // do something else
    }
    callback(result);
  });
}