How to Enable CORS for Firebase Cloud Functions?

Enabling CORS for Cloud Functions for Firebase

When attempting to access a Cloud Function for Firebase using an AJAX request, users may encounter the "No 'Access-Control-Allow-Origin'" error. This occurs due to the lack of CORS (Cross-Origin Resource Sharing) configuration. To rectify this issue, CORS must be enabled within the function.

Solution for Cloud Functions v2:

Cloud Functions v2 provides a straightforward solution by incorporating CORS into the function definition:

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

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

Solution for Cloud Functions v1:

For Cloud Functions v1, two sample functions provided by the Firebase team demonstrate CORS handling. The second sample employs a different method:

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

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

Additional Notes:

• If the function returns a 500 error code, Axios/jQuery will retry depending on the configuration. Change the status code to 200.
• Ensure the endpoint URL has the expected scheme (i.e. https://), matching the Origin header in the request.

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