node 远程下载文件

直接上代码：

    let fs = require('fs');
      let http = require("https");
      let url = "https://bpic.588ku.com/art_origin_min_pic/18/05/28/1518e5055708bad03daec3bceb3868af.jpg";
        http.get(url, function(res){
            let imgData = "";
            let contentLength = parseInt(res.headers['content-length']);
            //总长度    let fs = require('fs');
      let