Как правильно отправить кросс-доменный запрос?
 

Всем привет. Подскажите, пожалуйста, как решить следующую проблему.
Access to fetch at 'http://localhost:8080/feed/post' from origin 'http://localhost:3000' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.

Я отправляю данные на:
"http://localhost:8080/feed/post"

finishEditHandler = postData => {
  this.setState({
    editLoading: true
  });

  console.log('postData:', postData);

  // Set up data (with image!)
  const formData = new FormData();
  formData.append("title", postData.title);
  formData.append("content", postData.content);
  formData.append("image", postData.image);

  let url = "http://localhost:8080/feed/post";
  let method = "POST";
  // if (this.state.editPost) {
  //   url = 'URL';
  // }
  if (this.state.editPost) {
    url = "http://localhost:8080/feed/post/" + this.state.editPost._id;
    method = "PUT";
  }

  fetch(url, {
    method: method,
    body: formData,
    // headers: {
    //   'Content-Type': 'application/json'
    // },
    // body: JSON.stringify({
    //   title: postData.title,
    //   content: postData.content
    // })
  })
    .then(res => {
      if (res.status !== 200 && res.status !== 201) {
        throw new Error('Creating or editing a post failed!');
      }
      return res.json();
    })
    .then(resData => {
      const post = {
        _id: resData.post._id,
        title: resData.post.title,
        content: resData.post.content,
        creator: resData.post.creator,
        createdAt: resData.post.createdAt
      };
      this.setState(prevState => {
        let updatedPosts = [...prevState.posts];
        if (prevState.editPost) {
          const postIndex = prevState.posts.findIndex(
            p => p._id === prevState.editPost._id
          );
          updatedPosts[postIndex] = post;
        } else if (prevState.posts.length < 2) {
          updatedPosts = prevState.posts.concat(post);
        }
        return {
          posts: updatedPosts,
          isEditing: false,
          editPost: null,
          editLoading: false
        };
      });
    })
    .catch(err => {
      console.log(err);
      this.setState({
        isEditing: false,
        editPost: null,
        editLoading: false,
        error: err
      });
    });
};

Приложение у меня запущено на:
'http://localhost:3000'
Файл сервера выглядит следующим образом:

app.js

const path = require('path');

const express = require('express');
const bodyParser = require('body-parser');
const mongoose = require('mongoose');
const multer = require('multer');
const keys = require('./keys');

const feedRoutes = require('./routes/feed');

const app = express();

const fileStorage = multer.diskStorage({
  destination: (req, file, cb) => {
    cb(null, 'images');
  },
  filename: (req, file, cb) => {
    cb(null, new Date().toISOString() + '-' + file.originalname);
  }
});

const fileFilter = (req, file, cb) => {
  if (
    file.mimetype === 'image/png' ||
    file.mimetype === 'image/jpg' ||
    file.mimetype === 'image/jpeg'
  ) {
    cb(null, true);
  } else {
    cb(null, false);
  }
};

// app.use(bodyParser.urlencoded()); // x-www-form-urlencoded <form>
app.use(bodyParser.json()); // application/json

app.use('/images', express.static(path.join(__dirname, 'images')));
app.use(
  multer({storage: fileStorage, fileFilter: fileFilter}).single('image')
);
app.use((req, res, next) => {
  res.setHeader('Access-Control-Allow-Origin', '*');
  res.setHeader('Access-Control-Allow-Methods', 'OPTIONS, GET, POST, PUT, PATCH, DELETE');
  res.setHeader('Access-Control-Allow-Headers', 'Content-Type, Authorization');
  next();
});

app.use('/feed', feedRoutes);

app.use((error, req, res, next) => {
  console.log(error);
  const status = error.statusCode || 500;
  const message = error.message;
  res.status(status).json({
    message: message
  });
});

mongoose
  .connect(
    keys.mongoURI
  )
  .then(result => {
    console.log('MongoDB connected.');
    app.listen(8080);
  })
  .catch(err => console.log(err));

gsdev99, вот этот заголовок должен отдавать сервер, к которому обращаются (порт 8080), а не с которого отправляют запрос (порт 3000):

res.setHeader('Access-Control-Allow-Origin', '*');