title: firebase.auth is not a function - with multiple firebase app initialization
body: <p>I am getting the error 'Uncaught TypeError: firebase.auth is not a function'.</p>
  
  <p>My code was working perfectly, and then I implemented my clients api, and discovered that their api already uses and has initialized a different firebase app. So, as I couldn't initialise my firebase app in the usual way (ie. firebase.initializeApp(config) ), I followed the documentation on initializing multiple firebase apps as follows.  (This results in the error)</p>
  
  <pre><code>var configOther = {
  apiKey: "xxxx",
  authDomain: "xxxx",
  databaseURL: "xxxx",
  projectId: "xxxx",
  storageBucket: "xxxx",
  messagingSenderId: "xxxx"
  };
  
  var firebaseOther = firebase.initializeApp(configOther, "firebaseOther");
  
  console.log("auth: ", firebase.auth(firebaseOther)); //"Uncaught TypeError: firebase.auth is not a function"
  </code></pre>
  
  <p>Any ideas about what I'm doing wrong here? Many thanks.</p>
  
  <p>Note - I have also tried the following shorthand notation:</p>
  
  <pre><code>firebaseOther.auth() 
  </code></pre>
  
  <p>instead of </p>
  
  <pre><code>firebase.auth(firebaseOther)
  </code></pre>
  
  <p>but this results in the same error.</p>