How can I trigger a Cloud Function with only the changed Firestore fields?

No, there isn’t a built-in option to receive only the changed fields. When a Firestore document update triggers a Cloud Function, the function is provided with complete snapshots of both the previous and the current document states. This means you receive all fields in the document as part of the event data, not just the differences.

If you need only the changed data, you’ll have to compute a diff within your function by comparing the old (before) and new (after) snapshots. For example, in Node.js you could do something like this:

JSCopied
exports.updateUser = functions.firestore.document('users/{userId}')
  .onUpdate((change, context) => {
    const beforeData = change.before.data();
    const afterData = change.after.data();
    const diff = {};

    // Compare each key in the afterData
    Object.keys(afterData).forEach(key => {
      if (beforeData[key] !== afterData[key]) {
        diff[key] = { before: beforeData[key], after: afterData[key] };
      }
    });

    // Use the diff object as needed
    console.log('Changed fields:', diff);
  });

This manual diffing approach lets you extract only the fields that changed during the update.