The only important line of HomeScreen.js is line 34. The variable assignment here returns a Promise but no data.

Firebase.js

import * as firebase from 'firebase'
import 'firebase/firestore';

class FireBase {
  constructor() {
    let config = {
      apiKey: "AIzaSyAO8BB0W6mXDr3ihvkMn-uJjopgfsf_qow",
      authDomain: "cherishly-412dd.firebaseapp.com",
      databaseURL: "https://cherishly-412dd.firebaseio.com",
      projectId: "cherishly-412dd",
      storageBucket: "cherishly-412dd.appspot.com",
      messagingSenderId: "378187715149"
    }
     if (!firebase.apps.length) {
      firebase.initializeApp(config) 
    }
    const firestore = firebase.firestore();

    // Random thing required by firestore
    const settings = {timestampsInSnapshots: true};
    firestore.settings(settings);
  };

  // Retrieve data for specific collection and document
  downloadData = (collectionId, documentId) => {
    var ref = firebase.firestore().collection(collectionId).doc(documentId);

    ref.get().then(function(doc) {
      if (doc.exists) {
          console.log("Document data:", doc.data());
          return doc.data();
      } else {
          // doc.data() will be undefined in this case
          console.log("No such document!");
          return null;
      }
    }).catch(function(error) {
        console.log("Error getting document:", error);
        return null;
    });
  };

  // Retrieve all data within a collection
  downloadAllData = async (collectionId) => {
    var docList = [];
    var ref = firebase.firestore().collection(collectionId);
    // TODO: was missing a return so this function was returning null
    return ref.get().then(function(querySnapshot){
      querySnapshot.forEach(function(doc){
        var docDict = {}; 
        docDict["id"] = doc.id;
        docDict["data"] = doc.data();
        docList.push(docDict);
      });

      // The doc list prints to console here
      console.log(docList);
      return docList;
    });
   
  }


  uploadData = (collectionId, documentId, data) => {
    firebase.firestore().collection(collectionId).doc(documentId).set({
      data,
    })
  };
}

// Helper function to check if a list is empty
function isEmpty(obj) {
    for(var key in obj) {
        if(obj.hasOwnProperty(key))
            return false;
    }
    return true;
}


Fire = new FireBase();
export default Fire;

HomeScreen.js

import React, {Component} from 'react';
import { ScrollView, StyleSheet, View, Button, Text, TouchableHighlight } from 'react-native';

import Fire from '../components/firebase/Firebase';
import Navbar from '../components/navbar';
import AddButton from '../components/addbutton';
import PopupWindow from '../components/popupwindow';
import TestAddButton from '../components/testAddButton';

export default class HomeScreen extends Component {
	constructor(props) {
		super(props);
		this.handleAddPhotoButton = this.handleAddPhotoButton.bind(this);
		this.handleTestButtonPress = this.handleTestButtonPress.bind(this);
		this.data = [];
	}

	// Handle state of popup window here
	state = {
		popupWindow: false,
		loadingData: false, 
	};

	// Event handler for add photo button
	// Pass this handler to child component
	handleAddPhotoButton = (e) => {
		e.preventDefault();
		e.persist();
		this.setState(prevState => ({
  		popupWindow: !prevState.popupWindow
		}));


		var dataPromise = Fire.downloadAllData('users');
		// TODO: this now returns a promise (all await functions do) so wait for
		// `then` then you can use this.setState.
		// alternatively you can use the async/await syntax over `.then`
		dataPromise.then(data => {
			// Here the doc data doesn't print to console
			console.log(data);

			// Fire.downloadData("sample", "test15")
			// To upload to firebase
			// Fire.uploadData("sample", "test15", {"egf":"blah"});;
			console.log(this.state.popupWindow);
		});


	}

	handleTestButtonPress = (e) => {
		e.preventDefault();
		console.log(this.data);
	}

	// Removes extra whitespace introducedß by navigation
	static navigationOptions = {
    	header: null,
  };

	render() {
		const {navigate} = this.props.navigation;
		return (
			<View style={{flex: 1}}>
				<Navbar navigation={this.props.navigation} />

				<PopupWindow status={this.state.popupWindow} />

				<View style={styles.main}>
					<ScrollView>
						<Text>Cherishly is a photo sharing application!</Text>
					</ScrollView>
				</View>

				<AddButton handler={this.handleAddPhotoButton} />
				<TestAddButton handler={this.handleTestButtonPress} />
			</View>
		);
	}
}

const styles = StyleSheet.create({
	main : {
		flex: 9, 
		backgroundColor: 'white',
		justifyContent: 'center',
		alignItems: 'center'
	},
  addButton: {
  	position: 'absolute',
  	bottom: 30,
  	right: 30,
  	width: 80,
  	height: 80,
  	zIndex: 1,

  	justifyContent: 'center',
  	alignItems: 'center',
  	borderRadius: 50, 
  	backgroundColor: '#F35F64',
  }

})