Firebase | 从 Firebase 读取数据返回“未定义”，但 console.log 显示精确结果

Question

EDIT: I'm new to Stack, but why was my earlier thread locked? It just said "similar thread is found somewhere else" and the thread wasn't at all similar.

I am currently trying to update my own little personal weight tracker using Firebase and React Native. However, whenever I log DataSnapshot.val() I receive the input 22 , which is perfect. But when I return the very same value I receive Undefined .

I tried both get() and onValue() with the same results. The path is correct, since I get the correct data using console.log.

https://firebase.google.com/docs/database/web/read-and-write?authuser=2

I tried following the above documentation. But is it updated? snapshot is currently DataSnapshot ?

Firebase:

const readWeight = (database, userId) => {
  get(ref(database, `users/${userId}/weight`)).then((DataSnapshot) => {
    try {
      if (DataSnapshot.exists()) {
        console.log(
          `Weight found. Current weight is: ${DataSnapshot.val()} kg`
        );
        return DataSnapshot.val();
      } else {
        console.log("Weight wasn't found");
      }
    } catch (error) {
      console.log(error);
    }
  });
};

HomeScreen.js

// Modules
import { react, useState, useEffect } from "react";
import { Text, TouchableOpacity, View, TextInput, Image } from "react-native";
import { LogOutButton } from "../Components/LogOutButton";
import { auth, writeUserData, database, readWeight } from "../firebase";

// Stylesheet
import { styles } from "../Stylesheet/Stylesheet";

export const HomeScreen = () => {
  const user = auth.currentUser;

  let currentUserWeight = readWeight(database, user.uid);


  console.log("Current Weight: ", currentUserWeight);


  return (
    <View style={styles.profileMain}>
      <View style={styles.profileInfoContainer}>

        <View style={styles.HOME__profileWeightContainer}>

          <Text style={styles.HOME__profileWeightText}>
            Last Weight: {currentUserWeight}
          </Text>

        </View>
      </View>
      </View>
    </View>
  );
};

Answer 1

Data is loaded from Firebase asynchronously, and the return DataSnapshot.val() runs way after you actually call readWeight(database, user.uid) . Adding some logging is probably the best way to see that flow.

The solution is to store the weight in the state of your component with the useState hook.

export const HomeScreen = () => {
  const user = auth.currentUser;

  // 👇
  let { currentUserWeight, setCurrentUserWeight } = useState(0);

  useEffect(() => {
    const unsubscribe = get(ref(database, `users/${userId}/weight`)).then((DataSnapshot) => {
      if (DataSnapshot.exists()) {
        setCurrentUserWeight(DataSnapshot.val());
      }
    });  
    return () => unsubscribe();
  , []}
  // 👆

  return (
    <View style={styles.profileMain}>
      <View style={styles.profileInfoContainer}>

        <View style={styles.HOME__profileWeightContainer}>

          <Text style={styles.HOME__profileWeightText}>
            Last Weight: {currentUserWeight}
          </Text>

        </View>
      </View>
      </View>
    </View>
  );
};