Firestore setDoc() method somehow generates a unique identifier when doc says it won't

Question

Google documentation states that

When you use set() to create a document, you must specify an ID

However, I just learned that I CAN call setDoc without specifying the ID like this, and it will still generate a unique identifier.

await setDoc(doc(collctionRef), {
        createdAt: new Date(),
        updatedAt: new Date(),
      });

Question

1. Is the Firebase documentation outdated? or is there something I missed?
2. If this is true, addDoc seems useless now. Or is there another use for it?

Answer 1

If you don't specify document ID in doc(collctionRef) then it'll generate one automatically. The same was in previous SDK:

// This would add a Document with random ID

// V8
colRef.doc().set({})

// V9
setDoc(doc(colRef))

From the same documentation,

Behind the scenes, .add(...) and .doc().set(...) are completely equivalent, so you can use whichever is more convenient.