Why doesn't cross-domain fetch work?

Question

JavaScript

The issue is as follows, you need to send data to another domain. I have this implemented by 2 Node.js applications under different ports. On port 8080 (8081 - proxy) there is a UI server sharpened for isomorphic React ... On port 3000 there is an API server.

fetch('http://localhost:3000', {
  method: 'post',
  headers: {
    'Access-Control-Allow-Origin':'*',
     'Accept': 'application/json, text/plain, */*',
    'Content-Type': 'application/json'
  },
  mode: 'cors',
  body: JSON.stringify({a: 7, str: 'Some string: &=&'})
})
  .then(res=>res.json())
  .then(res => console.log(res));

The problem is that the "set" headers are not visible on the server.
If the request is sent to the same domain from which the page was loaded, then everything will be successful.

fetch('http://localhost:8080', {
  method: 'post',
  headers: {
    'Access-Control-Allow-Origin':'*',
     'Accept': 'application/json, text/plain, */*',
    'Content-Type': 'application/json'
  },
  mode: 'cors',
  body: JSON.stringify({a: 7, str: 'Some string: &=&'})
})
  .then(res=>res.json())
  .then(res => console.log(res));

As you can see in the photo, the headers are set correctly
. How to solve this problem? Thanks in advance!

Answer 1

Access-Control-Allow-Origin must be set not on the client, but on the server, on the server it is necessary to specify which domains can make a request, i.e. The response from the server must contain this header