ADN Authorization Code POST Request

example.js

app.get('/success', function(req, res){
  var postFunction = function(reqq) {
    var access_token = reqq.query.code;
    var post_data = querystring.stringify({
      'client_id': 'client_id',
      'client_secret': 'client_secret',
      'grant_type': 'authorization_code',
      'redirect_uri': 'http://127.0.0.1:3000/',
      'code': access_token
    });

    var options = {
      url: 'https://account.app.net/oauth/access_token?' + post_data,
      headers: {
        'Content-Type': 'application/x-www-form-urlencoded',
        'Content-Length': post_data.length
      }
    };
   var callback = function(error, response, body) {
      console.log(body);
      if (!error && response.statusCode == 200) {
        var info = JSON.parse(body);
      }
    };
    request.post(options, callback);
  };
  
  res.render('success', {}, postFunction(req));
})

'/success' is the redirect destination for the initial part of the Authorization flow.

This page is loading, but the POST returns a body containing:
'{"error_slug":"unsupported_grant_type","error":"Unknown grant type"}'

Ah, got it. You're sending the parameters as part of the URL, instead of the request body. It looks like you're using the request library, so the solution is quite simple:

app.get('/success', function(req, res){
  var postFunction = function(reqq) {
    var access_token = reqq.query.code;
    // request will encode this for you
    var post_data = {
      'client_id': 'client_id',
      'client_secret': 'client_secret',
      'grant_type': 'authorization_code',
      'redirect_uri': 'http://127.0.0.1:3000/',
      'code': access_token
    };

    var options = {
      // instead of concatting this to the URL, request
      // that it be encoded and sent as form data.
      // request will add the appropriate headers & encode for you
      url: 'https://account.app.net/oauth/access_token',
      form: post_data
    };
   var callback = function(error, response, body) {
      console.log(body);
      if (!error && response.statusCode == 200) {
        var info = JSON.parse(body);
      }
    };
    request.post(options, callback);
  };

  res.render('success', {}, postFunction(req));
})