from origin 'null' has been blocked by CORS policy code example

Example 1: has been blocked by CORS policy: Request header field content-type is not allowed by Access-Control-Allow-Headers in preflight response.

response.setHeader("Access-Control-Allow-Origin", "*");
response.setHeader("Access-Control-Allow-Credentials", "true");
response.setHeader("Access-Control-Allow-Methods", "GET,HEAD,OPTIONS,POST,PUT");
response.setHeader("Access-Control-Allow-Headers", "Access-Control-Allow-Headers, Origin,Accept, X-Requested-With, Content-Type, Access-Control-Request-Method, Access-Control-Request-Headers");

Example 2: 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 3: access to image at from origin

//use crossorigin="anonymous" sttribute in img tag

<img src="https://upload.wikimedia.org/wikipedia/commons/f/f9/Phoenicopterus_ruber_in_S%C3%A3o_Paulo_Zoo.jpg" id="external-flamingo" crossorigin="anonymous">