title: "Property 'map' does not exist on type '{}'" while trying to get collection from firebase firestore
body: <p>I have followed the documentation <a href="https://github.com/angular/angularfire2/blob/master/docs/firestore/collections.md" rel="nofollow noreferrer">here</a> to get data with document id from firestore.</p>
  
  <pre><code>  import { Injectable } from '@angular/core';
    import { AngularFirestore, AngularFirestoreCollection } from '@angular/fire/firestore';
    import { Observable } from 'rxjs';
    import { map } from 'rxjs/operators';
    import 'rxjs/add/operator/map';
  
    @Injectable()
    export class UserService {
    userCol : AngularFirestoreCollection&lt;UserInter&gt;;
    users : Observable&lt;UserInter[]&gt;;
    constructor(private afs:AngularFirestore) { } 
  
    GetUsers(){
      this.userCol = this.afs.collection&lt;UserInter&gt;('users');
      this.users = this.userCol.snapshotChanges().pipe(
                  map(changes =&gt;{
   error here-&gt;   return changes.map(a =&gt; {
                  const data = a.payload.doc.data() as UserInter;
                  data.id = a.payload.doc.id;
                  return data;
            })
          }))
  return this.users;  
  }
  
  export interface UserInter {
  email ?: string,
  Firstname ?: string,
  Lastname ?: string,
  Address ?: string,
  }
  
  export interface UserInterid extends UserInter {id ?: string }
  </code></pre>
  
  <p>When I ng-serve my application I get this error </p>
  
  <blockquote>
    <p>Property 'map' does not exist on type '{}'</p>
  </blockquote>