node写一个完整的前后端接口请求

后端：
1.新建一个文件夹
2.通过npm init创建package.json文件
3.通过 npm install express 安装express模块
4.在node文件夹中新建app.js文件
5.npm install mysql 连接mysql数据库

var express=require('express');
var app =express();
var mysql = require('mysql');
//设置跨域访问
app.all('*', function(req, res, next) {
	res.header("Access-Control-Allow-Origin", "*");
	res.header("Access-Control-Allow-Headers", "X-Requested-With");
	res.header("Access-Control-Allow-    Methods","PUT,POST,GET,DELETE,OPTIONS");
	res.header("X-Powered-By",' 3.2.1');
	res.header("Content-Type", "application/json;charset=utf-8");
	next();
});
var arr=[];
// 连接数据库
var connection = mysql.createConnection({
  host     : '121.22.124.225',
  user     : 'test',
  password : '123',
  database : 'rebuild'
});
connection.connect() 

//SQL语句查询
connection.query('SELECT * FROM hhyp_action_log LIMIT 20', function (error,         results, fields) {
  console.log('The solution is: ', results);
	arr = results
});

//写个接口123
app.get('/123',function(req,res){
	res.status(200),
	res.json(arr)
});
//配置服务端口
var server = app.listen(4000, function () {
	var host = server.address().address;
	var port = server.address().port;
	console.log('Example app listening at http://%s:%s', host, port);
})

前端：

var url = "http://127.0.0.1:4000/123"
 var xhr = new XMLHttpRequest()
 xhr.open('GET',url,true)
	xhr.send()
	xhr.onreadystatechange = function (e) {
	if (xhr.readyState == 4 && xhr.status == 200) {
	   console.log(xhr.responseText);
	}
};