后端node.js无法从前端接收数据，错误：发送后无法设置标头

Question

我正在构建一个应用程序，它从后端获取数据库中的数据并将数据发送到前端页面。从前端输入所需数据的命令，然后传递到后端，让后端从数据库中获取所需的数据。但是当前端POST命令到后端时，NODE控制台显示：

Error: Can't set headers after they are sent.

前端浏览器控制台显示：

POST http://localhost:5050/ net::ERR_EMPTY_RESPONSE

GET方法运行正常。所以后端和数据库之间的逻辑应该很好，我认为它可能是POST网址的问题？我测试了几个小时仍然无法解决它。请帮忙！提前致谢！

前端代码script.js：

$(document).ready(function() {
    var options = {
            chart: {
                    renderTo: containerSpline,
                    type: 'spline'
                },
            title: {
                text: '信息录入统计曲线图'
            },
            credits: {
                enabled: false
            },
            xAxis: {
                title: {
                    text: '时间'
                },
                categories: []
            },
            yAxis: {
                title: {
                    text: '数量'
                }
            },
            series: []
    };

    $.get("//localhost:5050", (data)=>{
        // feed the data into the chart
        var series = {
            name:'', 
            data: []
            };
        var xAxis = {
                categories: []
            };
        data.recordset.forEach((item)=>{
            series.data.push(item.Count);
            series.name = item.Dates;
            options.xAxis.categories.push(item.Times);
        })
        options.series.push(series);
        var chart = new Highcharts.Chart(options); 
    })

    $('button').click(()=>{
        var obj = {
            year: $('#year').val(),
            month: $('#month').val(),
            day: $('#day').val()
        }
        console.log(obj);
        $.post('//localhost:5050',JSON.stringify(obj), (text)=>{
            console.log(text); 
        })
    })
})

后端代码fetchData.js：

var express=require('express');
var http = require('http');
var sql=require('mssql');
var fs = require('fs');
var hbs = require('hbs');
var cors = require('cors');
var app=express();
var data;
//config for your database
var config={
        user:'elims',
        password:'abc123@ksd',
        server:'192.168.5.237',   
        database:'ELIMSDB'
    };
    //connect to your database
sql.connect(config,function (err) {
  if(err) console.log(err);
    //create Request object
  var request=new sql.Request();
  var fDate, lDate;
  fDate = '2017-08-04';
  lDate = '2017-08-05';
  request.input('firstdate', sql.NVarChar, fDate);
  request.input('lastdate', sql.NVarChar, lDate);
  request.query(" EXEC dbo.KSD_Front_YbidCountByMinutes @firstdate , @lastdate ", (err,recordset) => {
                      if(err) console.log(err);
                    //send records as a response
                      data = JSON.stringify(recordset);
                    });
  });
// send the data to the script.js
app.use(cors());
app.get('/',(req,res)=>{
    res.writeHead(200, {'Content-Type':'text/json;charset=utf-8'});
    res.end(data);
});
app.post('/', (req, res)=>{
  var text = 'Back end response!'
  console.log(req.params.obj);
  res.writeHead(200, {'Content-Type':'text/json;charset=utf-8'});
  res.send(text);
})
app.listen(5050,function(){
  console.log('Server is running!');
});