setDoc, Doc

To create or overwrite a single document, use the set() method:

react js/native

import { doc, setDoc } from "firebase/firestore"; 

// Add a new document in collection "cities"
await setDoc(doc(db, "cities", "LA"), {
  name: "Los Angeles",
  state: "CA",
  country: "USA"
});

node.js

const data = {
  name: 'Los Angeles',
  state: 'CA',
  country: 'USA'
};

// Add a new document in collection "cities" with ID 'LA'
const res = await db.collection('cities').doc('LA').set(data);

If the document does not exist, it will be created. If the document does exist, its contents will be overwritten with the newly provided data, unless you specify that the data should be merged into the existing document, as follows:

react js/native

import { doc, setDoc } from "firebase/firestore"; 

const cityRef = doc(db, 'cities', 'BJ');
setDoc(cityRef, { capital: true }, { merge: true });

node.js

const cityRef = db.collection('cities').doc('BJ');

const res = await cityRef.set({
  capital: true
}, { merge: true });