How to Configure CORS for Firebase Cloud Functions?

CORS Configuration in Cloud Functions for Firebase

When accessing Cloud Functions for Firebase through AJAX requests, you may encounter the "No 'Access-Control-Allow-Origin'" error indicating improper CORS configuration.

For Cloud Functions v2:

Firebase-functions/v2/https allows you to enable CORS directly within the function definition:

const { onRequest } = require("firebase-functions/v2/https");

exports.sayHello = onRequest({ cors: true }, (req, res) => {
  res.status(200).send("Hello world!");
});

For Cloud Functions v1:

Two Firebase-provided samples demonstrate the use of CORS:

• Time server with date formatting
• HTTPS endpoint requiring Authentication

The second sample employs a different approach than you've been using:

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

exports.fn = functions.https.onRequest((req, res) => {
  cors(req, res, () => {
    // Your function body here
  });
});

Steps to Resolve CORS Issues:

• If using Firebase Cloud Functions v1, ensure you've imported the cors module as shown in the sample code.
• Use cors(req, res, () => {}) to handle CORS preflight requests.
• Set origin: true within the cors() configuration to allow requests from any origin.
• If response codes like 500 or 403 are being returned, try setting the response status to 200.
• Verify that you're accessing the correct URL for your Cloud Function.

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