title: How to get document data from firebase.firestore.DocumentSnapshot?
body: <p>I get document from Firestore, it's work:</p>
  
  <pre><code>this.sub = this.beluginService.getBeluginById(this.personId)
  .subscribe((documentSnapshot: firebase.firestore.DocumentSnapshot) =&gt; {
    console.log(documentSnapshot.data());
  }
  </code></pre>
  
  <p>My service:</p>
  
  <pre><code>  getBeluginById(id: string) {
      return this.afs.collection('belugin').doc(id).get();
    }
  </code></pre>
  
  <p>In console.log I get one object: </p>
  
  <pre><code>{ id: "ya1jibU2pZx1niGiuAmp"
    qwCF: [0, 0, 2, 0, 0, 6, 2]
    qwCO: [0, 0, 0, 0, 2, 0, 0]
    qwIMP: [10, 0, 2, 2, 2, 4, 0]
    qwME: [0, 4, 0, 4, 2, 0, 0]
    qwPL: [0, 0, 0, 0, 0, 0, 0]
    qwRI: [0, 2, 2, 2, 2, 0, 0]
    qwSH: [0, 0, 0, 0, 0, 0, 0]
    qwTW: [0, 4, 4, 2, 2, 0, 8] }
  </code></pre>
  
  <p>But when I try to get object properties (etc id, qwCF):</p>
  
  <pre><code>this.sub = this.beluginService.getBeluginById(this.personId)
  .subscribe((documentSnapshot: firebase.firestore.DocumentSnapshot) =&gt; {
    this.id = documentSnapshot.data().id;
    this.qwCF = documentSnapshot.data().qwCF;
  }
  </code></pre>
  
  <p>I can't get data() properties.... My VCCode shows a method data() call error. Why?</p>