When app.use(cors()) is not working for you, try this snippet. Allows CORS for your application with Express.

fix-cors-express.js

// imports....
const app = express(); // create express app

// Set all headers that you need
const fixCors = (req, res, next) => {
    res.setHeader("Content-Type", "application/json,text/plain"); // Add content type if needed
    res.setHeader("Access-Control-Allow-Methods", "GET,PUT,POST,DELETE,PATCH,UPDATE,OPTIONS"); // All methods
    res.setHeader("Access-Control-Allow-Origin", "http://localhost:3000"); // Change origin if needed
    res.setHeader("Access-Control-Allow-Headers", "Origin, Access-Control-Allow-Origin, Content-Type, Accept, Authorization, Origin, Accept, X-Requested-With, Access-Control-Request-Method, Access-Control-Request-Headers");
    res.setHeader("Access-Control-Expose-Headers", "Origin, Access-Control-Allow-Origin, Content-Type, Accept, Authorization, Origin, Accept, X-Requested-With, Access-Control-Request-Method, Access-Control-Request-Headers");
    res.setHeader("Access-Control-Allow-Credentials", "true");
};

// Make sure this is the first `app.use`. Every request will go through this.
app.use((req, res, next) => { fixCors(req, res, next); next(); });
// For requests with OPTIONS, will approve the preflight.
app.options("*", (req, res, next) => { fixCors(req, res, next); console.log("send 200"); res.sendStatus(200); });

// Rest of your code.