获取： o'访问-控制-允许-起源'头存在于请求的资源：反应和节点

Question

我有这个错误，我不知道为什么在我的后端节点js我使用cors

const express = require('express');
const bodyParser = require('body-parser');
const port = 8080;
const  cors = require('cors');

require('./database/index')

const usersRouter = require('./routes/userRoutes');
const postRouter = require('./routes/post');

const app = express();
const router = express.Router();

usersRouter.configuration(router);
postRouter.configuration(router);

app.use(express.json());
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: true }));
app.set('port',port);
app.use('/', router);
app.use(cors());
app.use(function(req, res, next) {
    res.header("Access-Control-Allow-Origin", "*");
    res.header('Access-Control-Allow-Methods: GET, POST, OPTIONS');
    res.header("Access-Control-Allow-Headers", "Origin, X-Requested-With, Content-Type, Accept");
    next();
  });

我的方法前端：

signIn = () => {
    const data = {login:this.login,password:this.password};
    const requestInfo = {
        method:'POST',
        body: JSON.stringify({data}),
        headers: new Headers({
            'Content-Type': 'application/json'
        }),
    };
    fetch('http://localhost:8080/login', requestInfo)
    .then(response => {
        if(response.ok){
            return response.json();
        }
        throw new Error("Login Invalido..")
    })
    .then(token => {
        sessionStorage.setItem('token', JSON.stringify(token.token));
        this.props.history.push("/users");
        return;
    })
    .catch(e => {
        this.setState({message: e.message})
    });
}

错误：

CORS策略阻止了从来源http://localhost:3000获取http://localhost:8080/login的访问：对起飞前请求的响应不通过权限改造检查：请求的资源上没有访问控制允许来源标头。如果不透明的响应满足您的需求，请将请求的模式设置为no-cors，以在禁用CORS的情况下获取资源。

Answer 1

尝试将CORS置于路由器配置之前。

const express = require('express');
const bodyParser = require('body-parser');
const port = 8080;
const  cors = require('cors');

require('./database/index')

const usersRouter = require('./routes/userRoutes');
const postRouter = require('./routes/post');

const app = express();
const router = express.Router();

app.use(cors());
app.use(function(req, res, next) {
    res.header("Access-Control-Allow-Origin", "*");
    res.header('Access-Control-Allow-Methods: GET, POST, OPTIONS');
    res.header("Access-Control-Allow-Headers", "Origin, X-Requested-With, Content-Type, Accept");
    next();
  });

app.use(express.json());
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: true }));
app.set('port',port);

usersRouter.configuration(router);
postRouter.configuration(router);

app.use('/', router);