PROBLEM Getting a single field in a document in Firebase Firestore - firebase

Question

I'm trying to do a Cloud Function in Firebase. Initially, I read my document. Then I would take two fields "a" and "b", from this document. Then I want to set another field of my doc ('rank') as the sum of 'a' and 'b'. I can't find a solution!
I want to take only the fields "a" and "b" from my document. Save them in variables. And use those variables to do the sum, and set the result in "rank" field.
I tried:
var data = doc.data()
a = data.a
b = data.b
but it doesn't work.
Code:
export const daicazzo = functions.https.onRequest((request,response)=>{
const store = admin.firestore();
//var b;
store.collection('questions').doc('LD92BBDOihAC3fHDyoV').get().then(doc =>{
if(doc.exists){
response.send(doc.data())
}
else{
response.send("Nothing")
}
}).catch(reason => {
console.log(reason)
response.send(reason)
})
store.collection('questions').doc('LD92BBDOihAC3fHDyoV').set({
rank: //a+b
})
.then(function() {
console.log("done");
})
.catch(function(error){
console.log("Error:",error);
});
});

Answer 1

Should do the work (might be some better solutions)
const store = admin.firestore();
export const daicazzo = functions.https.onRequest(async (request,response)=>{
const questionRef = store.doc(`questions/${LD92BBDOihAC3fHDyoV}`)
const doc = await questionRef.get()
const foundDoc = doc.exists
if (foundDoc) {
// getting both key/value pairs from doc object
const {a, b} = doc.data()
const rank = a + b // whatever is your logic...
// saving the rank in the same document
await questionRef.update({rank}) // or questionRef.set({rank}, {merge: true})
if you want to send back the doc with the updated rank without making another read, assuming you handle errors:
const { id } = doc
const updatedDoc = { ...doc.data(), id, rank }
return response.send(updatedDoc)
} else {
return response.send("Nothing")
}
});