Firestore - How to get document id after adding a document to a collection

Question

Is there a way to acquire the document id that was generated after adding a document to a collection?

If I add a document to a collection that represents a \"post\"

Answer 1

If using promises, I'd recommend using fat arrow function as it opens up the possibility for using this.foo even in the .then function

db.collection("cities").add({
    name: "Tokyo",
    country: "Japan"
})
.then(docRef => {
    console.log("Document written with ID: ", docRef.id);
    console.log("You can now also access this. as expected: ", this.foo)
})
.catch(error => console.error("Error adding document: ", error))

Using function(docRef) means you cannot access this.foo, and error will be thrown

.then(function(docRef) {
    console.log("Document written with ID: ", docRef.id);
    console.log("You can now NOT access this. as expected: ", this.foo)
})

While fat arrow functions will allow you to access this.foo as expected

.then(docRef => {
    console.log("Document written with ID: ", docRef.id);
    console.log("You can now also access this. as expected: ", this.foo)
})

---

Edit/addition 2020:

A more popular way these days may be to use async/await instead. Notice that you have to add async in front of the function declaration:

async function addCity(newCity) {
  const newCityAdded = await db.collection("cities").add(newCity)
  console.log("the new city:", newCityAdded)
  console.log("it's id:", newCityAdded.id)
}

And if you only want the id it can be grabbed using descructuring. Destructuring allows you to grab any key/value-pair in the response:

async function addCity(newCity) {
  const { id } = await db.collection("cities").add(newCity)
  console.log("the new city's id:", id)
}

It's also possible to use destructuring to grab the value and rename to whatever you want:

async function addCity(newCity) {
  const { id: newCityId } = await db.collection("cities").add(newCity)
  console.log("the new city's id:", newCityId)
}