title: ReferenceError: facebook is not defined : can't connect with facebook login React Native
body: <p>Im starting with React native and i tried to use expo-facebook to connect users (im following a tutoriel on youtube)</p>
  <p>in the end a get this message : ReferenceError: facebook is not defined</p>
  <p>**AuthScreen
  ../user/jobs/screen/users/AuthScreen.js:10
  7 | // create a component
  8 | const AuthScreen = (props) =&gt; {
  9 |</p>
  <blockquote>
  <p>10 |        facebook = async () =&gt; {
  11 |     const {type, token}  =   await Facebook.logInWithReadPermissionsAsync(&quot;xxxx&quot;, {
  12 |                                                             permission:&quot;public_profile&quot;
  13 |                                                         })**</p>
  </blockquote>
  <p>this is the code :</p>
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="true">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>import React, { Component } from 'react';
  import { View, Text, StyleSheet, Button } from 'react-native';
  import * as Facebook from 'expo-facebook'
  import * as firebase from 'firebase'
   
  const AuthScreen = (props) =&gt; {
  
         facebook = async () =&gt; {
      const {type, token}  =   await Facebook.logInWithReadPermissionsAsync("xxxx", {
                                                              permission:"public_profile"
        
        })
        
         if(type == "success"){
             const credential =  firebase.auth.FacebookAuthProvider.credential(token)
  
             firebase.auth().signInWithCredential(credential)
             .catch(error =&gt; {
                 console.log(error)
             })
         }
  
         props.navigation.navigate('Jobs')
  
         }
  
      return (
          &lt;View style={styles.container}&gt;
           &lt;Text  style={{fontSize: 35, textAlign:'center', paddingBottom:8}}&gt;{`Find Jobs or \nSubmit Jobs`}&lt;/Text&gt;
           &lt;View style={{paddingHorizontal:30, marginTop:10}} &gt;
               &lt;Button title="Login with Facebook" onPress={facebook} /&gt;
           &lt;/View&gt;
          &lt;/View&gt;
      );
  }; 
   
  export default AuthScreen;</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.6.3/umd/react.production.min.js"&gt;&lt;/script&gt;
  &lt;script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.6.3/umd/react-dom.production.min.js"&gt;&lt;/script&gt;</code></pre>
  </div>
  </div>
  </p>
  <p>thank you</p>