Enabling CORS in Cloud Functions for Firebase

Handling specific HTTP methods

CORS policies and configurations might require different treatment depending on the HTTP method in use. Let us illustrate for GET, POST, and OPTIONS methods:

const cors = require('cors')({origin: true});
exports.yourFunction = functions.https.onRequest((req, res) => {
  cors(req, res, () => {
    switch (req.method) {
      case 'GET':
        // GET: like asking politely at the front desk.
        res.status(200).send("Hello World via GET");
        break;
      case 'POST':
        // POST: like sliding your application under the door.
        res.status(200).send("Hello World via POST");
        break;
      case 'OPTIONS':
        // OPTIONS: like peeping to see if anyone's home before entry.
        res.status(204).send('');
        break;
      default:
        // Default: like knocking on the wrong door.
        res.status(405).send({error: "Invalid method. Clearly, you're lost!"});
        break;
    }
  });
});
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Investigating common errors

CORS errors are like pesky flies, not always the culprit though can often mimic that role. Error 500 might indicate other underlying server issues. Let's illuminate solutions for common cases:

• Keep an eye on Firebase function logs for detailed error messages like a detective on coffee.
• Assure your code is clear from syntax and runtime errors - like double-checking your shopping list.
• Manually set CORS headers if needed with response.set('Access-Control-Allow-Origin', '*').

Going around CORS with Firebase hosting rewrites

If you want to play hide and seek with CORS and go ninja, Firebase hosting rewrites might just be your camouflage to bypass CORS entirely. Here's a firebase.json snippet:

{
  "hosting": {
    "rewrites": [
      {
        "source": "/yourFunction",
        "function": "yourFunction"
      }
    ]
  }
}


Directing requests via Firebase hosting, you cunningly bypass CORS, while the browser perceives requests of the same origin.

CORS in Typescript: a guide

In the Typescript playground, the game rules are slightly different. Do not forget to install types for the cors library:

npm install @types/cors --save-dev
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Follow the same approach as in JavaScript, but remember all features are kept under check with strong typing and the right compiler configurations.

Further considerations

• Keeping up to date: Stay updated on Firebase updates impacting CORS handling.
• Community & documentation: Balance weight between these two for enriched knowledge.
• Error management: Concurrently deal with CORS errors as well as unforeseen server errors.