Access to XMLHttpRequest at 'localhost:8000' from origin 'http://localhost:3000' has been blocked by CORS policy: code example

Example: Access to XMLHttpRequest at from origin HTTP localhost:3000 has been blocked by CORS policy

const cors = require('cors');
const corsOptions ={
    origin:'http://localhost:3000', 
    credentials:true,            //access-control-allow-credentials:true
    optionSuccessStatus:200
}
app.use(cors(corsOptions));