Resolving the TypeError: Cannot read properties of undefined in Next.js with Firebase

---

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---

Understanding the Problem

The error message you’re encountering relates to an attempt to use the .map() function on an undefined variable. This typically happens when the variable you're trying to map over isn't an array or is simply not defined at the time of execution.

In your code, you attempted to use the getDoc() function from Firebase to retrieve a collection of messages. However, getDoc() is designed to fetch a single document from Firestore, rather than an array of documents, which is likely what you intended in this situation. This discrepancy leads to your app trying to map over an undefined value, hence the error.

The Code You're Dealing With

[[See Video to Reveal this Text or Code Snippet]]

The issues arise because messagesRes is not an array, causing the subsequent mapping to fail.

Implementing the Solution

To resolve this issue, you should use the getDocs() function instead of getDoc(). The getDocs() function retrieves all documents in a collection, allowing you to loop through them with .map() as needed.

Step-by-Step Fix

Update your import statement: Ensure you have imported getDocs from Firebase Firestore.

[[See Video to Reveal this Text or Code Snippet]]

Modify the getServerSideProps function: Change the call from getDoc() to getDocs().

Here’s the updated version of your code:

[[See Video to Reveal this Text or Code Snippet]]

Key Takeaways

Use getDocs() for Multiple Documents: Remember that getDocs() is used to fetch multiple documents from a collection, which is exactly what you want when mapping through messages.

Check for Undefined Values: Always ensure that the values you're trying to access and manipulate are defined and of the expected type to avoid runtime errors.

Conclusion