CORS错误，我已经搜索过，但是找不到。 Node.js

Question

const express = require('express');
const mongoose = require('mongoose');
const bodyParser = require('body-parser')
const cors = require('cors');
User = require('../server/models/userListModel');

const app = express();
app.use(cors());
app.use(bodyParser.urlencoded({ extended: false }));
app.use(bodyParser.json());

mongoose.connect('mongodb://localhost:27017/Userdb', {useNewUrlParser: true})

const routes = require('../server/routes/userListRoutes');
routes(app);

app.listen(process.env.PORT || 3000);
console.log('Connected to Express/MongoDB Server')

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我有cors，但在创建时仍然出现此错误 无法加载htttp：// localhost：3000 / users：仅协议方案支持跨源请求：http，数据，chrome，chrome扩展名，https。