title: Firebase Callable Function + CORS
body: <p>I'm trying to call a callable Cloud Function from my app, but I'm facing CORS issues. 
  <a href="https://i.stack.imgur.com/QO8rs.png" rel="noreferrer"><img src="https://i.stack.imgur.com/QO8rs.png" alt="enter image description here"></a></p>
  
  <p>I can't enable Cors since I don't have access to the request and response on the onCall function. This is my function:</p>
  
  <pre><code>exports.UserInvitation = functions.https.onCall((data, context) =&gt; {
    const email = data.email
  
  
    return new Promise((resolve, reject) =&gt; {
      admin.auth().createUser({
        email: email,
        emailVerified: false,
        password: password
      }).then(resolve).catch((err) =&gt; {
        console.error(err.code)
        reject(new functions.https.HttpsError(err.code, err.message))
      })
    })
  })
  </code></pre>
  
  <p>And this is how I call it:</p>
  
  <pre><code>functions.httpsCallable('UserInvitation')({ email: this.input.value }).then((data) =&gt; {
        console.log('Sent invitation:', data)
  })
  </code></pre>
  
  <p>Firebase-functions package.json:</p>
  
  <pre><code>{
    "name": "functions",
    "description": "Cloud Functions for Firebase",
    "scripts": {
      "serve": "firebase serve --only functions",
      "shell": "firebase functions:shell",
      "start": "npm run shell",
      "deploy": "firebase deploy --only functions",
      "logs": "firebase functions:log"
    },
    "dependencies": {
      "bluebird": "^3.5.1",
      "firebase-admin": "~5.12.0",
      "firebase-functions": "^1.0.1"
    },
    "private": true
  }
  </code></pre>
  
  <p>WEB SDK Firebase version: 4.13.1</p>