Backend and Frontend running on different port, CORS error

To resolve the CORS error in the browser you should add the following HTTP header to the response:

Access-Control-Allow-Headers: Content-Type

You can do that by adding the following code:

app.use(cors({  'allowedHeaders': ['Content-Type'],  'origin': '*',  'preflightContinue': true}));

Just my two cents...

1. Allow the frontend as AllowedOrigin
2. Set allowCredentials to true.
3. Do not use a wildcard (*) for AllowedOrigin (again, if you are dealing with cookies/authentication). Use protocol, host AND port [Why].

A Golang example (using gorilla/handlers):

handlers.CORS(    // allowCredentials = true    handlers.AllowCredentials(),    // Not using TLS, localhost, port 8080     handlers.AllowedOrigins([]string{"http://localhost:8080"}),    handlers.AllowedMethods([]string{"GET", "POST", "PUT", "HEAD", "OPTIONS"}),    handlers.AllowedHeaders([]string{"X-Requested-With", "Content-Type", "Authorization"}),)

use a CORS middleware like

var enableCORS = function (req, res, next) {  res.header('Access-Control-Allow-Origin', '*');  res.header('Access-Control-Allow-Methods', 'GET,PUT,POST,DELETE,OPTIONS');  res.header('Access-Control-Allow-Headers', 'Content-Type, token, Content-Length, X-Requested-With, *');  if ('OPTIONS' === req.method) {    res.sendStatus(200);  } else {    next();  }};app.all("/*", function (req, res, next) {  res.header('Access-Control-Allow-Origin', '*');  res.header('Access-Control-Allow-Methods', 'GET,PUT,POST,DELETE,OPTIONS');  res.header('Access-Control-Allow-Headers', 'Content-Type, token, Content-Length, X-Requested-With, *');  next();});app.use(enableCORS);