title: Generate 6 digit unique number in Firebase
body: <p>I want to generate 6 digit <strong>unique</strong> number in Firbase (unique must)
  <a href="https://i.stack.imgur.com/I3yqg.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/I3yqg.png" alt="enter image description here"></a></p>
  
  <p>above attendeeCode should be unique on every new attendess entry</p>
  
  <p>I am new in firebase and I have searched alot but could not find </p>