angular - What is snapshotchanges in firestore

Question

this function is called by the constructor. can someone explain in depth to me what it does?

  initializeItems(){
this.travelList$ = this.plsdala.getTravelList()
.snapshotChanges()
.map(
  changes => {
    return changes.map(c=>({
      key: c.payload.key, ...c.payload.val()
    })).slice().reverse();

   //to reverse order
  });

}

Answer 1

Waitting for answers