Answers for "CORS policy: No 'Access-Control-Allow-Origin' header"

cross origin even with allow header

header('Access-Control-Allow-Origin: http://localhost:8100');
header ("Access-Control-Expose-Headers: Content-Length, X-JSON");
header ("Access-Control-Allow-Methods: GET, POST, PATCH, PUT, DELETE, OPTIONS");
header ("Access-Control-Allow-Headers: Content-Type, Authorization, Accept, Accept-Language, X-Authorization");
header('Access-Control-Max-Age: 86400');

if ($_SERVER['REQUEST_METHOD'] === 'OPTIONS') {
    // The request is using the POST method
    header("HTTP/1.1 200 OK");
    return;

}

Access to XMLHttpRequest at 'http://localhost/MySQL_pracs/InsertUser.php' from origin 'http://localhost:4200' has been blocked by CORS policy: Request header field content-type is not allowed by Access-Control-Allow-Headers in preflight response.

//Access to XMLHttpRequest at 'http://localhost/[api path].php' from origin 'http://localhost:4200' has been blocked by CORS policy: Request header field content-type is not allowed by Access-Control-Allow-Headers in preflight response.

//The error is simply saying that "Content-Type" is missing from "Access-Control-Allow-Headers".

//Therefore we need to add "Content-Type" to "Access-Control-Allow-Headers".

<?php 
header('Access-Control-Allow-Headers: Content-Type');
-----
?>

Reason: CORS header 'Access-Control-Allow-Origin' missing

const app = express();

// CORS configuration
const corsOptions = {
    origin: 'http://localhost:3000',
    credentials: true
}

// The following is not needed, CORS middleware will be applied
// using the Apollo Server's middleware API (see further below)
// app.use(cors(corsOptions))

// Setup JWT authentication middleware
app.use(async (req, res, next) => {
    const token = req.headers['authorization'];
    if(token !== "null"){
        try {
            const currentUser = await jwt.verify(token, process.env.SECRET)
            req.currentUser = currentUser
        } catch(e) {
            console.error(e);
        }
    }
    next();
});

const server = new ApolloServer({ 
    typeDefs, 
    resolvers, 
    context: ({ req }) => ({ Property, User, currentUser: req.currentUser })
});

// There is no need to explicitly define the 'path' option in
// the configuration object as '/graphql' is the default endpoint
// If you planned on using a different endpoint location,
// this is where you would define it.
server.applyMiddleware({ app, cors: corsOptions });

const PORT = process.env.PORT || 4000;

app.listen(PORT, () => {
    console.log(`Server listening on ${PORT}`);
})