Firebase: How to retrieve value from database then modify it and generate a new document with that value?

Question

I am trying to retrieve the postId value of the last document created. I retrieved it but I could not find a way to bring that value out of the query. The problem with my code is that the variable that I am querying is trapped inside the promise and I am stuck on finding how to extract it.

exports.postOnePost = (req, res) => {
  const first = db
    .collection("posts")
    .orderBy("createdAt", "desc")
    .limit(1)
    .get()
    .then(function (documentSnapshots) {
      const lastPostId = documentSnapshots.docs[
        documentSnapshots.docs.length - 1
      ].get("postId");
      console.log(lastPostId); // logs 10257
    });

I wanted to take lastPostId and increment it by one when a new post is added. When I added console.log(lastVisible) outside the function, it is not defined.

  const newPost = {
    createdAt: new Date().toISOString(),
    postId: lastPostId++, // This does not work
    body: req.body.body,
  };

  db.collection("posts")
    .add(newPost)
    .then((doc) => {
      const resPost = newPost;
      resPost.docId = doc.id;
      res.json(resPost);
    })
    .catch((err) => {
      res.status(500).json({ error: "something went wrong" });
      console.error(err);
    });
};

My database documents look like this:

{
      body: { stringValue: 'this is my body', valueType: 'stringValue' },
      createdAt: {
        stringValue: '2020-09-28T15:48:42.903Z',
        valueType: 'stringValue'
      },
      postId: { integerValue: '10257', valueType: 'integerValue' },
}

Answer 1

exports.createPost = (req, res) => {
  db.collection("posts")
    .orderBy("createdAt", "desc")
    .limit(1)
    .get()
    .then(function (documentSnapshots) {
      let lastDocId = documentSnapshots.docs[
        documentSnapshots.docs.length - 1
      ].get("postId");

      if (req.body.body.trim() === "") {
        return res.status(400).json({ body: "Body must not be empty" });
      }

      lastDocId++;

      const newPost = {
        createdAt: new Date().toISOString(),
        postId: lastDocId,
        body: req.body.body,
      };

      db.collection("posts")
        .add(newPost)
        .then((doc) => {
          const resPost = newPost;
          resPost.docId = doc.id;
          res.json(resPost);
        })
        .catch((err) => {
          res.status(500).json({ error: "something went wrong" });
          console.error(err);
        });
    })
    .catch((err) => {
      res.status(500).json({ error: "something went wrong" });
      console.error(err);
    });
};