CORS Middleware (Higher Order Function) for firebase function // Just Nice Syntax

withCORS.js

//For firebase funtion.
/*
Usage:

exports.foo = functions.https.onRequest(withCORS((req, res) => {
    res.send("Hello World");
});

*/

const withCORS = next => (req, res) => {
  res.header('Access-Control-Allow-Origin', '*'); // Allowed Origins.
  res.header('Access-Control-Allow-Headers', '*'); // Allowed Headers.
  res.header('Access-Control-Expose-Headers', 'token'); // Exposed Headers - means client only can access those headers.

  if (req.method === 'OPTIONS') {
    res.header('Access-Control-Allow-Methods', 'PUT, POST, PATCH, DELETE'); // Allowed Methods.
    res.header('Access-Control-Max-Age', 5 * 24 * 60 * 60); //5 days... But Chrome will take its MAX value.  :D
    return res.status(200).json({});
  }

  next(req, res);
};

module.exports = { withCORS };