title: What is snapshotchanges in firestore
body: <p>this function is called by the constructor. can someone explain in depth to me what it does?</p>
  
  <pre><code>  initializeItems(){
  this.travelList$ = this.plsdala.getTravelList()
  .snapshotChanges()
  .map(
    changes =&gt; {
      return changes.map(c=&gt;({
        key: c.payload.key, ...c.payload.val()
      })).slice().reverse();
  
     //to reverse order
    });
  
  }
  </code></pre>