deletePhoto.js

export const deletePhoto = functions.firestore
  .document("posts/{postId}")
  .onUpdate((change, context) => {
    // Get an object representing the document
    const updatedPost = change.after.data() as any;

    // ...or the previous value before this update
    const oldPost = change.before.data() as any;

    const oldImages: string[] = oldPost.images;
    const newImages: string[] = updatedPost.images;

    const deletedImages = oldImages.filter(oldImage => {
      return !newImages.some(newImage => newImage === oldImage);
    });

    const bucket = firebase.storage().bucket();

    const imagesRemovePromises = deletedImages.map((imagePath: string) => {
      return bucket.file(imagePath).delete();
    });

    return Promise.all(imagesRemovePromises);
  });