title: Firestore - Listen to specific field change?
body: <p>How can I listen to a specific field change with firestore js sdk ?</p>
  
  <p>In the documentation, they only seem to show how to listen for the whole document, if any of the "SF" field changes, it will trigger the callback.</p>
  
  <pre><code>db.collection("cities").doc("SF")
  .onSnapshot(function(doc) {
      console.log("Current data: ", doc &amp;&amp; doc.data());
  });
  </code></pre>