title: Call Firebase HTTPS callable cloud functions in React
body: <p>So, I'm working on a React project which uses Firebase to achieve lots of functionalities.
  And now I'm trying to use some HTTPS callable functions in it.</p>
  <p>But it seems like the way I import the 'firebase/functions' module is not correct. And it's giving me this error:</p>
  <pre><code>TypeError: Cannot read property 'httpsCallable' of undefined
  </code></pre>
  <p>Below is how I do the import and set up:</p>
  <pre><code>import app from 'firebase/app';
  import 'firebase/auth';
  import 'firebase/firestore';
  import 'firebase/functions';
  
  const config = {
     // the config info here
  };
  
    class Firebase {
      constructor() {
        app.initializeApp(config);
        this.auth = app.auth();
        this.db = app.firestore();
        this.functions = app.functions();
      }
  
      // trying to call the function
      doCreatePlanner = this.functions.httpsCallable('createPlanner')
  </code></pre>
  <p>Can anyone point me to the right direction?</p>