FireStore create a document if not exist

I think you want to use the following code:

db.collection('users').doc(user_id).set({foo:'bar'}, {merge: true})

This will set the document with the provided data and will leave other document fields intact. It is best when you're not sure whether the document exists. Simply pass the option to merge the new data with any existing document to avoid overwriting entire documents.

For more detailed about managing data with firestore check this link

If you need things like created and updated timestamps, you can use this technique:

let id = "abc123";
let email = "[email protected]";
let name = "John Doe";
let document = await firebase.firestore().collection("users").doc(id).get();
if (document && document.exists) {
  await document.ref.update({
    updated: new Date().toISOString()
  });
}
else {
  await document.ref.set({
    id: id,
    name: name,
    email: email,
    created: new Date().toISOString(),
    updated: new Date().toISOString()
  }, { merge: true });
}

This will create the document if it doesn't exist with created and updated timestamps, but only change the updated timestamp if it exists.