访问node服务器跨域，post请求变成了options,设置请求头后，获取不到post请求提交的数据

var http = require('http');
var url = require("url");

http.createServer(function (req, res) {
    var urlObj = url.parse(req.url);
    var pathname = urlObj.pathname;
    // 关闭nodejs 默认访问 favicon.ico
    if (!pathname.indexOf('/favicon.ico')) {
        return; 
    };
     let requestM=req.method;

    res.writeHeader(200,{
        "access-control-allow-origin": "*",
        "access-control-allow-methods": "GET, POST, PUT, DELETE, OPTIONS",
        "access-control-allow-headers": "Content-Type,Content-Length, Authorization, Accept,X-Requested-With",
        "access-control-max-age": 10,
        "Content-Type": "application/json"
    });

    if(requestM==="POST"){
        var post = '';  
                // 通过req的data事件监听函数，每当接受到请求体的数据，就累加到post变量中
                req.on('data', function(chunk){    
                    post += chunk;
                });
    }
    //获取不到post 请求过来的参数

}).listen(3000);