这是我得到的错误：CORS策略已阻止从源“http：//localhost：3000”访问位于“http：//localhost：5050/api/posts/”的XMLHttpRequest：请求的资源上不存在“Access-Control-Allow-Origin”标头。
这是我的索引。我的express API的js代码：

const express = require('express');
require('dotenv').config();
const { errorHandler } = require('./middleware/errorMiddleware');
require('colors');
const connectDb = require('./config/dbConnect');
const PORT = process.env.PORT || 5050;
const cors = require('cors');

// Connect DB
connectDb();

// Routes
const inviteRoutes = require('./routes/inviteRoutes');
const userRoutes = require('./routes/userRoutes');
const postRoutes = require('./routes/postRoutes');

const app = express();

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

const corsOptions = {
  origin: 'http://localhost:3000',
};
app.use(cors(corsOptions));

app.use('/api/invite', inviteRoutes);
app.use('/api/users', userRoutes);
app.use('/api/posts', postRoutes);

app.use(errorHandler);

app.listen(PORT, () => {
  console.log(`Server started on port: ${PORT}`);
});