Enabling CORS in Cloud Functions for Firebase

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" }
]

No CORS solutions worked for me... till now!

Not sure if anyone else ran into the same issue I did, but I set up CORS like 5 different ways from examples I found and nothing seemed to work. I set up a minimal example with Plunker to see if it was really a bug, but the example ran beautifully. I decided to check the firebase functions logs (found in the firebase console) to see if that could tell me anything. I had a couple errors in my node server code, not CORS related, that when I debugged released me of my CORS error message. I don't know why code errors unrelated to CORS returns a CORS error response, but it led me down the wrong rabbit hole for a good number of hours...

tl;dr - check your firebase function logs if no CORS solutions work and debug any errros you have

I have a little addition to @Andreys answer to his own question.

It seems that you do not have to call the callback in the cors(req, res, cb) function, so you can just call the cors module at the top of your function, without embedding all your code in the callback. This is much quicker if you want to implement cors afterwards.

exports.exampleFunction = functions.https.onRequest((request, response) => {
    cors(request, response, () => {});
    return response.send("Hello from Firebase!");
});

Do not forget to init cors as mentioned in the opening post:

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

Update: Any response function that takes time risk a CORS error with this implementation because this doesn't have the appropriate async/await. Don't use outside of quick prototyping endpoints that return static data.

Updated answer: using cors library with Typescript support:

install cors

npm i -S cors
npm i --save-dev @types/cors

index.ts:

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

// allow cors in http function
export const myFunction = functions.https.onRequest((req, res) => {
corsHandler(req, res, async () => {

// your method body

 });
});

Old answer: (not working anymore)
Found a way to enable cors without importing any 'cors' library. It also works with Typescript and tested it in chrome version 81.0.

exports.createOrder = functions.https.onRequest((req, res) => {
// browsers like chrome need these headers to be present in response if the api is called from other than its base domain
  res.set("Access-Control-Allow-Origin", "*"); // you can also whitelist a specific domain like "http://127.0.0.1:4000"
  res.set("Access-Control-Allow-Headers", "Content-Type");

  // your code starts here

  //send response
  res.status(200).send();
});

This might be helpful. I created firebase HTTP cloud function with express(custom URL)

const express = require('express');
const bodyParser = require('body-parser');
const cors = require("cors");
const app = express();
const main = express();

app.post('/endpoint', (req, res) => {
    // code here
})

app.use(cors({ origin: true }));
main.use(cors({ origin: true }));
main.use('/api/v1', app);
main.use(bodyParser.json());
main.use(bodyParser.urlencoded({ extended: false }));

module.exports.functionName = functions.https.onRequest(main);

Please make sure you added rewrite sections

"rewrites": [
      {
        "source": "/api/v1/**",
        "function": "functionName"
      }
]

If you don't/can't use cors plugin, calling the setCorsHeaders() function first thing in the handler function will also work.

Also use the respondSuccess/Error functions when replying back.

const ALLOWED_ORIGINS = ["http://localhost:9090", "https://sub.example.com", "https://example.com"]


// Set CORS headers for preflight requests
function setCorsHeaders (req, res) {
  var originUrl = "http://localhost:9090"


  if(ALLOWED_ORIGINS.includes(req.headers.origin)){
    originUrl = req.headers.origin
  }

  res.set('Access-Control-Allow-Origin', originUrl);
  res.set('Access-Control-Allow-Credentials', 'true');

  if (req.method === 'OPTIONS') {
    // Send response to OPTIONS requests
    res.set('Access-Control-Allow-Methods', 'GET,POST','PUT','DELETE');
    res.set('Access-Control-Allow-Headers', 'Bearer, Content-Type');
    res.set('Access-Control-Max-Age', '3600');
    res.status(204).send('');
  }
}

function respondError (message, error, code, res) {
  var response = {
    message: message,
    error: error
  }
  res.status(code).end(JSON.stringify(response));
}


function respondSuccess (result, res) {
  var response = {
    message: "OK",
    result: result
  }
  res.status(200).end(JSON.stringify(response));
}

Simple solution using the Google Cloud Console Dashboard:

Go to your GCP console dashboard:

https://console.cloud.google.com/home/dashboard

Go to menu

"Cloud Functions" ("Compute" section)

Select your cloud function, e.g. "MyFunction", a side menu should appear on the right showing you the access control settings for it Click on "Add Member", type in "allUsers" and select the role "Cloud Function Invoker" Save it -> now, you should see a remark "Allow unauthenticated" in the list of your cloud functions

Access is now available to everybody from the internet with the correct config to your GCP or Firebase project. (Be careful)

If there are people like me out there: If you want to call the cloud function from the same project as the cloud function it self, you can init the firebase sdk and use onCall method. It will handle everything for you:

exports.newRequest = functions.https.onCall((data, context) => {
    console.log(`This is the received data: ${data}.`);
    return data;
})

Call this function like this:

// Init the firebase SDK first    
const functions = firebase.functions();
const addMessage = functions.httpsCallable(`newRequest`);

Firebase docs: https://firebase.google.com/docs/functions/callable

If you can't init the SDK here is the essence from the other suggestions:

If you use firebase hosting and host in the default location, choose rewrites: https://firebase.google.com/docs/hosting/full-config#rewrites

Or use CORS like krishnazden suggested: https://stackoverflow.com/a/53845986/1293220

Only this way works for me as i have authorization in my request:

exports.hello = functions.https.onRequest((request, response) => {
response.set('Access-Control-Allow-Origin', '*');
response.set('Access-Control-Allow-Credentials', 'true'); // vital
if (request.method === 'OPTIONS') {
    // Send response to OPTIONS requests
    response.set('Access-Control-Allow-Methods', 'GET');
    response.set('Access-Control-Allow-Headers', 'Content-Type');
    response.set('Access-Control-Max-Age', '3600');
    response.status(204).send('');
} else {
    const params = request.body;
    const html = 'some html';
    response.send(html)
} )};

A cors error can occur if you don't catch an error in a function. My suggestion is to implement a try catch in your corsHandler

const corsHandler = (request, response, handler) => {
    cors({ origin: true })(request, response, async () => {
        try {
            await handler();
        }
        catch (e) {
            functions.logger.error('Error: ' + e);
            response.statusCode = 500;
            response.send({
                'status': 'ERROR' //Optional: customize your error message here
            });
        }
    });
};

Usage:

exports.helloWorld = functions.https.onRequest((request, response) => {
    corsHandler(request, response, () => {
        functions.logger.info("Hello logs!");
        response.send({
            "data": "Hello from Firebase!"
        });
    });
});

Thanks to stackoverflow users: Hoang Trinh, Yayo Arellano and Doug Stevenson

I have just published a little piece on that:

https://mhaligowski.github.io/blog/2017/03/10/cors-in-cloud-functions.html

Generally, you should use Express CORS package, which requires a little hacking around to meet the requirements in GCF/Firebase Functions.

Hope that helps!

Changing true by "*" did the trick for me, so this is how it looks like:

const cors = require('cors')({ origin: "*" })

I tried this approach because in general, this is how this response header is set:

'Access-Control-Allow-Origin', '*'

Be aware that this will allow any domain to call your endpoints therefore it's NOT secure.

Additionally, you can read more on the docs: https://github.com/expressjs/cors

For what it's worth I was having the same issue when passing app into onRequest. I realized the issue was a trailing slash on the request url for the firebase function. Express was looking for '/' but I didn't have the trailing slash on the function [project-id].cloudfunctions.net/[function-name]. The CORS error was a false negative. When I added the trailing slash, I got the response I was expecting.

Go into your Google Cloud Functions. You may have not seen this platform before, but it's how you'll fix this Firebase problem. Find the Firebase function you're searching for and click on the name. If this page is blank, you may need to search for Cloud Functions and select the page from the results. Find your function, click on the name. Go to the permissions tab. Click Add (to add user). Under new principles, type 'allUsers' -- it should autocomplete before you finish typing. Under select a role, search for Cloud Functions, then choose Invoker. Save. Wait a couple minutes.

This should fix it. If it doesn't, do this AND add a CORS solution to your function code, something like:

  exports.sendMail = functions.https.onRequest((request, response) => {
  response.set("Access-Control-Allow-Origin", "*");
  response.send("Hello from Firebase!");
});

If You are not using Express or simply want to use CORS. The following code will help resolve

const cors = require('cors')({ origin: true, });   
exports.yourfunction = functions.https.onRequest((request, response) => {  
   return cors(request, response, () => {  
        // *Your code*
    });
});

If you're testing firebase app locally then you need to point functions to localhost instead of cloud. By default, firebase serve or firebase emulators:start points the functions to server instead of localhost when you use it on your web app.

Add below script in html head after firebase init script:

 <script>
      firebase.functions().useFunctionsEmulator('http://localhost:5001')
 </script> 

Make sure to remove this snippet when deploying code to server.

I got the error because I was calling a function that didn't exist on the client side. For example:

firebase.functions().httpsCallable('makeSureThisStringIsCorrect');

Adding my piece of experience. I spent hours trying to find why I had CORS error.

It happens that I've renamed my cloud function (the very first I was trying after a big upgrade).

So when my firebase app was calling the cloud function with an incorrect name, it should have thrown a 404 error, not a CORS error.

Fixing the cloud function name in my firebase app fixed the issue.

I've filled a bug report about this here https://firebase.google.com/support/troubleshooter/report/bugs

From so much searching, I could find this solution in the same firebase documentation, just implement the cors in the path:

import * as express from "express";
import * as cors from "cors";


const api = express();
api.use(cors({ origin: true }));
api.get("/url", function);

Link firebase doc: https://firebase.google.com/docs/functions/http-events

If you prefer to make a single handler function (reference answer)

const applyMiddleware = handler => (req, res) => {
  return cors(req, res, () => {
    return handler(req, res)
  })
}
exports.handler = functions.https.onRequest(applyMiddleware(handler))

I'm a very beginner with Firebase (signed up 30 minutes ago). My issue is that I called my endpoint

https://xxxx-default-rtdb.firebaseio.com/myendpoint

Instead of

https://xxxx-default-rtdb.firebaseio.com/myendpoint.json

If you just started with Firebase, make sure you don't forget the .json extension.

I have been trying this for a long time.

It finally finally worked when I made this change.

app.get('/create-customer', (req, res) => {
  return cors()(req, res, () => {
    ... your code ...

The Big difference is that I used cors()(req, res... instead of directly cors(req, res...

It Now works perfectly.

With the same access allow control origin error in the devtool console, I found other solutions with also more modern syntax :

My CORS problem was with Storage (and not RTDB neither the browser...), and then I'm not in possession of a credit card (as requested by the aforementioned solutions), my no-credit card solution was to :

install gsutil : https://cloud.google.com/storage/docs/gsutil_install#linux-and-macos to create a cors.json file to be loaded via terminal with gsutil

gsutil cors set cors.json gs://[ your-bucket ]/-1.appspot.com

https://firebase.google.com/docs/storage/web/download-files#cors_configuration

In my case the error was caused by cloud function invoker limit access. Please add allUsers to cloud function invoker. Please catch link. Please refer to article for more info

If none of the other solutions work, you could try adding the below address at the beginning of the call to enable CORS - redirect:

https://cors-anywhere.herokuapp.com/

Sample code with JQuery AJAX request:

$.ajax({
   url: 'https://cors-anywhere.herokuapp.com/https://fir-agilan.web.app/gmail?mail=asd@gmail.com,
   type: 'GET'
});

See below for how I set up my Express with CORS.

The 'https://pericope.app' is my custom domain for my Firebase project.

It looks like all other answers recommend origin:true or *.

I'm hesitant to allow all origins since it would allow anyone else access to the api. That's fine if you are creating a public service, but if you're doing anything with your data it is risky since it is a privileged environment. For example, this admin SDK bypasses any security rules you have setup for Firestore or Storage.

//Express
const express = require('express');
const app = express();

const cors = require('cors');
app.use(cors({
  origin: 'https://pericope.app'
}));

Use cors on your https.onRequest with Typescript like this:

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

export const pingFunctionWithCorsAllowed = functions.https.onRequest((request, response) => {
  corsHandler(request, response, () => {
    response.send(`Ping from Firebase (with CORS handling)! ${new Date().toISOString()}`);
  });
});

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