Failed to connect blocked by cors

This code gives me an err: 'Failed to connect blocked by cors’, but im not using cors in my server.

full error message:

Access to fetch at 'http://localhost:3000/newPerevrences' from origin 'http://localhost:3001' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: 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.

async function changePereferences(newPereferences) {
    
    const response = await fetch('http://localhost:3000/newPereferences', {
        method: 'POST',
        body: JSON.stringify(newPereferences),
        headers: {
          'Content-Type':'application/json',
          'Access-Control-Allow-Origin':'*',
          'Access-Control-Allow-Methods':'POST,PATCH,OPTIONS'
        }

    });

    const resData = await response.json();

    if(!response.ok) {
        throw new Error('Failed to update perevrences')
    };

    return resData.message;
}

This function is in a separate file but I'm calling it in my App.jsx

Server Packages:

{
  "name": "backend",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "dependencies": {
    "express": "^4.18.2"
  }
}

server.js:

const express = require('express');
const port = 3000;

const fs = require('fs');


const app = express();

app.use(express.json());

//paths
app.put('/newPereferences', async (req, res) => {
    const newPerevrences = req.body.newPerevrences;

    await fs.writeFile('./perevrences.json', JSON.stringify(newPerevrences));

    res.status(200).json({ message: 'Perevrences updated successfully!'})
})

app.listen(port, () => {

})

I added the headers: {'Content-Type':'application/json', 'Access-Control-Allow-Origin':'*', 'Access-Control-Allow-Methods':'POST,PATCH,OPTIONS'} but it didn't solve my problem.

Just use

const cors = require("cors");
app.use(cors())

everything will work fine.

For more details visit https://expressjs.com/en/resources/middleware/cors.html