Access to XMLHttpRequest at 'https://**' from origin 'https://**' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource. code example

Example 1: 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');
-----
?>

Example 2: Access to XMLHttpRequest has been blocked by CORS policy React

/*
npm i cors
or
 yarn add cors
then in your node app
*/
const cors = require('cors');
const corsOptions ={
    origin:'http://localhost:3000', 
    credentials:true,            //access-control-allow-credentials:true
    optionSuccessStatus:200
}
app.use(cors(corsOptions));