title: i want key value when there is change in it's child firebase and javascript
body: <p>structure of my firebase db is like below image
  <a href="https://i.stack.imgur.com/xuWOh.png" rel="nofollow noreferrer">image of my firebase DB stucture</a></p>
  
  <p>whenever thare is new message it will be added under particular id like in image it is "59a004ce9bfedb3f50ff2ec5" id
  so every new message will create new node under perticuler id's goup_messages.</p>
  
  <p>i want to get perticuler id whenever there is new message in it.</p>
  
  <p>i have use </p>
  
  <pre><code>var getvalue = firebase.database().ref('foram-ec60c/' +userId+ '/goup_massages/');
          getvalue.once('value', function(snapshot) {});
      }
  </code></pre>
  
  <p>but i don't know how to get value of id?</p>
  
  <p>i have use javascript to add data in firebase DB</p>