how to deploy app to heroku and proxy API request

Can you help me find what's wrong when I try to deploy my fullstack app to heroku. where can I find the solution to make my http requests work ?

I have tried :

• adding a proxy in the package.json
• using npm http-proxy-middleware

Nothing seems to work ??

This is my index.js

const express = require("express");
const bodyParser = require("body-parser");
require('dotenv').config();
const app = express();
const { createProxyMiddleware } = require("http-proxy-middleware");

const path = require("path");
const routes = express.Router()
const {  getAll, getSingle, postArticle, postComment, postVote } = require("./routes");

const cors = require("cors");
app.use(cors());
app.use('/articles', routes);
routes.use("/",  getAll);
routes.use("/:id", getSingle)
routes.use("/:id/upvote", postVote);
routes.use("/insert", postArticle);
// create another collection comment
routes.use("/:id/add-comment", postComment);

//middleware
app.use(bodyParser.urlencoded({ extended: true }));
app.use(bodyParser.json());


// Step 1:
app.use(express.static(path.resolve(__dirname, "./build")));
// Step 2:


app.get('*', function(req, res) {
  res.sendFile(path.join(__dirname, './build/index.html'));
});

app.listen(4000, () => {
  console.log(`server running on port 4000`);
});

and, package.json

{
  "name": "app",
  "version": "0.1.0",
  "private": true,
  "dependencies": {
    "@testing-library/jest-dom": "^5.16.4",
    "@testing-library/react": "^13.1.1",
    "@testing-library/user-event": "^13.5.0",
    "react": "^18.0.0",
    "react-dom": "^18.0.0",
    "react-scripts": "5.0.1",
    "web-vitals": "^2.1.4"
  },
  "scripts": {
    "start": "react-scripts start",
    "build": "react-scripts build",
    "test": "react-scripts test",
    "eject": "react-scripts eject"
  },
  "eslintConfig": {
    "extends": [
      "react-app",
      "react-app/jest"
    ]
  },
  "browserslist": {
    "production": [
      ">0.2%",
      "not dead",
      "not op_mini all"
    ],
    "development": [
      "last 1 chrome version",
      "last 1 firefox version",
      "last 1 safari version"
    ]
  }
}

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