How can I solve the error 'TS2532: Object is possibly 'undefined'?

Question

I'm trying to rebuild a web app example that uses Firebase Cloud Functions and Firestore. When deploying a function I get the following error:

src/index.ts:45:18 - error TS2532: Object is possibly 'undefined'.
45     const data = change.after.data();

This is the function:

export const archiveChat = functions.firestore
  .document("chats/{chatId}")
  .onUpdate(change => {
    const data = change.after.data();

    const maxLen = 100;
    const msgLen = data.messages.length;
    const charLen = JSON.stringify(data).length;

    const batch = db.batch();

    if (charLen >= 10000 || msgLen >= maxLen) {

      // Always delete at least 1 message
      const deleteCount = msgLen - maxLen <= 0 ? 1 : msgLen - maxLen
      data.messages.splice(0, deleteCount);

      const ref = db.collection("chats").doc(change.after.id);

      batch.set(ref, data, { merge: true });

      return batch.commit();
    } else {
      return null;
    }
  });

I'm just trying to deploy the function to test it. And already searched the web for similar problems, but couldn't find any other posts that match my problem.

Answer 1

As of October 2019, optional chaining (the ? operator) is now available on TypeScript 3.7 (Beta). You may install that version by running the following command:

npm install typescript@beta

As such, you can simplify your expression to the following:

const data = change?.after?.data();

You may read more about it from the release notes, which cover other interesting features released on that version.

Update (as of November 2019)

TypeScript's optional chaining is now officially available. Installing the latest version of typescript should allow you to access the cool new features.

npm install typescript

That being said, Optional Chaining can be used alongside Nullish Coalescing to provide a fallback value when dealing with null or undefined values

const data = change?.after?.data() ?? someOtherData();