access to xmlhttprequest at from origin has been blocked by cors policy code example

Example 1: javascript access to xmlhttprequest at from origin 'null' has been blocked by cors policy

//Open the HTML file using live server, it will work

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

/*
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));

Example 3: ccess to XMLHttpRequest at 'http://127.0.0.1:5000/ has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource.

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