express使用cros开启跨域访问

Access to XMLHttpRequest at 'http://127.0.0.1:8080/' 
from origin 'null' has been blocked by CORS policy: 
No 'Access-Control-Allow-Origin' header is present on the requested resource.

├── index.html
├── index.js
└── package.json

{
  "dependencies": {
    "cors": "^2.8.5",
    "express": "^4.17.1"
  }
}

var express = require('express')
var cors = require('cors')
var app = express()
app.use(cors())
app.get('/', function (req, res) {
    res.send("hello")
})
app.listen(8080, function () {
    console.log('listening: http://127.0.0.1:8080/')
})

<script>
    var request = new XMLHttpRequest();
    request.open('GET', 'http://127.0.0.1:8080', true)
    request.send(null)
    request.onreadystatechange = function () {
        if (request.readyState == 4 && request.status == 200) {
            console.log(request.responseText);
        }
    }
</script>