cors issue on github oauth

import request from 'superagent';

const self = this;
    request
      .post('https://github.com/login/oauth/access_token')
      .set('Content-Type', 'multipart/form-data')
      .query({
        client_id: CLIENT_ID,
        client_secret: CLIENT_SECRET,
        callback: 'http://127.0.0.1:3000/callback',
        code,
        state,
      })
      .end((err, res) => {
        const token = res.body.access_token;
        console.log(token);
        self.setToken(token);
      });

The code above will give me an error like this

XMLHttpRequest cannot load https://github.com/login/oauth/access_token?client_id=112asdecf3805fdada12&…127.0.0.1%3A3000%2Fcallback&code=434ebd7bb98d9809bf6e&state=HelloWorld1234. No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://127.0.0.1:3000' is therefore not allowed access.

I have no idea why even though I've registered the oauth application with github and callback url is http://127.0.0.1:3000/callback

While all the actual GitHub API endpoints support CORS by sending the right response headers, it is a known issue that the https://github.com/login/oauth/access_token endpoint for creating an OAuth access token does not support CORS requests from Web applications.

The very specific workaround for this case is to use https://github.com/prose/gatekeeper:

Gatekeeper: Enables client-side applications to dance OAuth with GitHub.

Because of some security-related limitations, Github prevents you from implementing the OAuth Web Application Flow on a client-side only application.

This is a real bummer. So we built Gatekeeper, which is the missing piece you need in order to make it work.

The general workaround is: Use an open reverse proxy like https://cors-anywhere.herokuapp.com/

var req = new XMLHttpRequest();
req.open('POST',
  'https://cors-anywhere.herokuapp.com/https://github.com/login/oauth/access_token',
  true);
req.setRequestHeader('Accept', 'application/json');
req.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
req.send('code=' + encodeURIComponent(location.query.code) +
    '&client_id=foo' +
    '&client_secret=bar');
...

See also How to use Cors anywhere to reverse proxy and add CORS headers.