1const cors = require('cors');
2const corsOptions ={
3    origin:'http://localhost:3000', 
4    credentials:true,            //access-control-allow-credentials:true
5    optionSuccessStatus:200
6}
7app.use(cors(corsOptions));

1/*
2npm i cors
3or
4 yarn add cors
5then in your node app
6*/
7const cors = require('cors');
8const corsOptions ={
9    origin:'http://localhost:3000', 
10    credentials:true,            //access-control-allow-credentials:true
11    optionSuccessStatus:200
12}
13app.use(cors(corsOptions));

1//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.
2
3//The error is simply saying that "Content-Type" is missing from "Access-Control-Allow-Headers".
4
5//Therefore we need to add "Content-Type" to "Access-Control-Allow-Headers".
6
7<?php 
8header('Access-Control-Allow-Headers: Content-Type');
9-----
10?>

1@app.route('your route', methods=['GET'])
2def yourMethod(params):
3    response = flask.jsonify({'some': 'data'})
4    response.headers.add('Access-Control-Allow-Origin', '*')
5    return response

1app.use(function (request, response, next) {
2  response.header("Access-Control-Allow-Origin", "*");
3  response.header("Access-Control-Allow-Headers", "Origin, X-Requested-With, Content-Type, Accept");
4  next();
5});

1app.use(function (req, res, next) {
2    //Enabling CORS
3    res.header("Access-Control-Allow-Origin", "*");
4    res.header("Access-Control-Allow-Methods", "GET,HEAD,OPTIONS,POST,PUT");
5    res.header("Access-Control-Allow-Headers", "Origin, X-Requested-With, Content-Type, 
6    Accept, x-client-key, x-client-token, x-client-secret, Authorization");
7      next();
8    });