Cloud function - update post like count

updatePostLikeCount.ts

export const updatePostLikeCount = functions.firestore
    .document('posts/{postId}/likes/{uid}')
    .onWrite((change, context) => {

        const postId = context.params.postId;

        let increment: number;

        const publicPostRef = firestore.collection('posts').doc(postId);

        //onCreate
        if (change.after.exists && !change.before.exists) {
            increment = 1;
            //onDelete
        } else if (!change.after.exists && change.before.exists) {
            increment = -1;
        } else {
            return null;
        }

        return publicPostRef.set({
            'like_count': admin.firestore.FieldValue.increment(increment)
        }, { merge: true });

    });