Encountering an issue with displaying Firestore timestamps while utilizing onSnapshot() in React Native results in an error

Currently, I'm in the process of developing a chat application using Firestore. The approach involves utilizing Flatlist and querying with onSnapshot() to provide real-time updates for both sender and receiver.

Here's an example of my query:

const ChatMessages = useCallback(() => {
 privateMessage
  .doc(chatId)
  .collection('messages')
  .orderBy('createdAt', 'asc')
  .onSnapshot((querySnapshot) => {
    const messageList = [];
    querySnapshot.forEach((doc) => {
      messageList.push(doc.data());
    })
    setMessages(messageList);
  }, error => {
    console.log(error)
  })
 }, [chatId])

In the sample flatlist, I include the user's display name, message content, and the timestamp when the message was created:

<FlatList
  data={messages}
  keyExtractor={(item, index) => String(index)}
  removeClippedSubviews={false}
  renderItem={({ item }) => (

   <View>

      <Text style={chatstyle.pmdate}>
        {item.createdAt.toDate().toDateString()}
      </Text>

      <Text>
        {item.displayName}
      </Text>

      <Text>
        {item.message}
      </Text>

   </View>
  )}
/>

Although I aim to show the date and time stamp alongside the message, encountering an issue where the line {item.createdAt.toDate().toDateString()} throws a null object error while using onSnapshot(). Any advice on the correct method to display the timestamp through onSnapshot() without errors?

Your input would be greatly appreciated!

Answer №1

The issue you are experiencing is related to "latency compensation": "Local writes in your application will trigger snapshot listeners immediately. ... When you perform a write, your listeners will receive notification of the updated data before it is sent to the backend". Refer to the documentation on onSnapshot().

By utilizing

firebase.firestore.FieldValue.serverTimestamp()
to assign the value of createdAt (which is a recommended practice), the value of createdAt is actually determined by the backend (the serverTimestamp marker is substituted with a server-generated timestamp in the saved data).

Hence, when the snapshot listener is activated in your frontend after the local write ("Local writes in your app will invoke snapshot listeners immediately"), this specific value remains unset (item.createdAt.toDate() triggers an error).

One potential solution involves using the metadata.hasPendingWrites property that denotes if the document contains local modifications that are pending submission to the backend.

For instance:

const ChatMessages = useCallback(() => {
 privateMessage
  .doc(chatId)
  .collection('messages')
  .orderBy('createdAt', 'asc')
  .onSnapshot((querySnapshot) => {
    const messageList = [];
    querySnapshot.forEach((doc) => {
      messageList.push(doc.data());
    })
    if (!querySnapshot.metadata.hasPendingWrites) {  // <======
       setMessages(messageList);
    }

  }, error => {
    console.log(error)
  })
 }, [chatId])

Answer №2

As a newcomer to React, I recently encountered a challenge and eventually found a quick solution for those who may be facing a similar issue:

const ChatMessages = useCallback(() => {
 privateMessage
  .doc(chatId)
  .collection('messages')
  .orderBy('createdAt', 'asc')
  .onSnapshot((querySnapshot) => {
    const messageList = [];
    querySnapshot.forEach((doc) => {
     if(querySnapshot.createdAt) // This is the crucial line of code that made all the difference
      messageList.push(doc.data());
    })
    setMessages(messageList);
  }, error => {
    console.log(error)
  })
 }, [chatId])

Just like that, by adding a single if statement to check for the createdAt attribute in the data, you can filter out unnecessary data from being added to your array.

If you are concerned about any drawbacks to this approach, one thing to consider is the extra time it takes for the validation within the if() statement in your program.

For those who attempted the previously recommended solution on their react application but still faced issues with null timestamps, know that you are not alone.

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