我知道这是一个常见的问题，有很多解决方案，但是我尝试了所有的方法，什么都没有改变。我在Heroku上部署了node和postgresql，以拥有一个Rest API，并通过HttpClient从Angular中获取它。我已经部署了它，在Postman中一切都正常，但是在浏览器中，它显示了这个错误：

CORS策略已阻止从源“http：//localhost：4200”访问位于“https：//myapi.herokuapp.com/products/”的XMLHttpRequest：请求的资源上不存在“Access-Control-Allow-Origin”标头。

这是我的节点应用程序：

const express = require('express');
const cors = require('cors');
const app = express();

//MiddleWares
app.use(express.json());
app.use(express.urlencoded({extended:false}));

//Router:
app.use(require('./routes/index'));

const PORT = process.env.PORT || 4000;

const corsOptions = {origin: process.env.URL || '*', credentials: true};

app.use(cors(corsOptions));

app.use((req, res, next) => {
    res.header("Access-Control-Allow-Origin", "*")
    res.header(
      "Access-Control-Allow-Headers",
      "Origin, X-Requested, Content-Type, Accept Authorization"
    )
    if (req.method === "OPTIONS") {
      res.header(
        "Access-Control-Allow-Methods",
        "POST, PUT, PATCH, GET, DELETE"
      )
      return res.status(200).json({})
    }
    next()
  });

app.listen(PORT, err => {
    if(err) throw err;
    console.log('Server on port 4000');
});

这是我的包裹

{
  "name": "backend",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "start": "node src/index.js"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "dependencies": {
    "cors": "^2.8.5",
    "express": "^4.18.1",
    "pg": "^8.7.3"
  },
  "devDependencies": {
    "nodemon": "^2.0.16"
  }
}

和Angular ：

constructor(
    private http: HttpClient
  ) { }

  getAllProducts() {
    return this.http.get<Product[]>(`${environment.url_api}/products/`);
  }

浏览器扩展没有帮助，因为我需要将Angular项目部署在托管