Javascript 为什么我在ExpressJS中的cors在一种情况下出错，而在另一种情况下却没有？

我正在建立一个数据库并将其连接到一些路由。我设法让它在一条路线上运行，但在另一条路线上却不行。“注册”路由有效，但“登录”路由无效。当我尝试运行登录路径时，会出现“cors错误”

我已尝试使用console.logs查看问题所在。我还尝试将cors添加到我的节点服务器

    const app = express();
    app.use(cors());

下面是实际工作的“/register”路由的后端代码

app.post('/register', (req, res) => {
 const { email, password } = req.body;

MongoClient.connect(url, function(err, db) {
  if (err) throw err;
  var dbo = db.db("expense_tracker");
  var myobj = {
    email: email,
    password: password,
    budget: "100",
    spent: "0",
    expenses: [],
    food: "0",
    clothing: "0",
    personal: "0",
    entertainment: "0",
    other: "0"

  };
  dbo.collection("users").insertOne(myobj, function(err, response) {
    if (err) throw err;
    console.log("1 document inserted");
    console.log(response.ops);

    if(email !== '' || password !== ''){
        res.json(response.ops);
    }else{
        res.status(400).json("One of the fields is blank; couldn't return user");
    }
    db.close();
  });
});
})

下面是它连接到我的react应用程序的位置：

onSubmitRegister = () => {
 console.log(this.state.signinPassword);
    fetch('https://server-budget-blaze349.c9users.io/register', {
        method: 'post',
        headers: {'Content-Type': 'application/json'},
        body: JSON.stringify({
            email: this.state.signinEmail,
            password: this.state.signinPassword
        })
    }).then(response => response.json())
    .then(user => {
      console.log('user', user);
        if(user){
              this.props.onRouteChange('budget');
              this.props.loadUser(user[0]);
        }
    })
   }  

我在请求时收到一个工作JSON对象

但是，当对登录执行类似操作时：

app.post('/signin', (req, res) => {
 const { email, password } = req.body;

console.log("FRONT END : " + password + ", " + email);

console.log("BACK END: ", database[0].password);

MongoClient.connect(url, function(err, db) {
  if (err) throw err;
  var dbo = db.db("expense_tracker");
  var query = { email: email, password: password };
  dbo.collection("users").find(query).toArray(function(err, result) {
    if (err) throw err;
    console.log(result);
    res.json(result.ops[0]);
    db.close();
  });
});



})

前端：

   onSubmitSignIn = () => {
       console.log(this.state.signinPassword);
    fetch('https://server-budget-blaze349.c9users.io/signin', {
        method: 'post',
        headers: {'Content-Type': 'application/json'},
        body: JSON.stringify({
            email: this.state.signinEmail,
            password: this.state.signinPassword
        })
    }).then(response => response.json())
    .then(user => {
      console.log('user', user);
        if(user){
          this.props.loadUser(user);
              this.props.onRouteChange('budget');
        }
    })
  }  

我得到这个错误：

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.

我一直在想为什么会这样，但我还是不太确定。对于如何修复此“cors”错误，如有任何帮助，将不胜感激

谢谢

替换

应用程序。使用（cors（））

替换

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

无需删除

cors（）

插件，请参见此答案