Enabling CORS in Cloud Functions for Firebase

Question

There are two sample functions provided by the Firebase team that demonstrate the use of CORS:

• Time server with date formatting
• HTTPS endpoint requiring Authentication

The second sample uses a different way of working with cors than you're currently using.

Consider importing like this, as shown in the samples:

const cors = require('cors')({origin: true});

And the general form of your function will be like this:

exports.fn = functions.https.onRequest((req, res) => {
    cors(req, res, () => {
        // your function body here - use the provided req and res from cors
    })
});

You can set the CORS in the cloud function like this

response.set('Access-Control-Allow-Origin', '*');

No need to import the cors package

For anyone trying to do this in Typescript this is the code:

import * as cors from 'cors';
const corsHandler = cors({origin: true});

export const exampleFunction= functions.https.onRequest(async (request, response) => {
       corsHandler(request, response, () => {});
       //Your code here
});

One additional piece of info, just for the sake of those googling this after some time: If you are using firebase hosting, you can also set up rewrites, so that for example a url like (firebase_hosting_host)/api/myfunction redirects to the (firebase_cloudfunctions_host)/doStuff function. That way, since the redirection is transparent and server-side, you don't have to deal with cors.

You can set that up with a rewrites section in firebase.json:

"rewrites": [
        { "source": "/api/myFunction", "function": "doStuff" }
]