google cloud function - firestore query snapshot size return undefined

I have a google function here with storage onFinalize trigger does 2 things in firestore db

1. add the element details into collection elements
2. get the size of the element collection

Why is my snapshot.size return undefined?

Console log snapshot returns the correct array result.

exports.elementChange = functions.storage.object().onFinalize((object) => {


    var element = {
        name: object.name,
        time: object.updated
    }


    db.collection('elements').doc(object.name).set(element)
    .then(db.collection('elements').get())
    .then((snapshot) => {

        console.log(snapshot);

        console.log("current size is "+snapshot.size);

        return console.log('element total count updated in db'); 

    })
    .catch(err => (console.log('Error when finalise element: '+ err)))

    // return console.log('finished finalise element');

})