关于nodejsiconv的编码无效说明

var fs = require("fs");
var iconv=require("iconv-lite");
var request=require("request");
var txt=fs.readFileSync("./a.txt","utf8");
iconv.decode(txt,"gbk")

request("http://jd.com",function(err,res,body){
 console.log(iconv.decode(body,"gbk"))
});

var fs = require("fs");
var iconv=require("iconv-lite");
var request=require("request");
var txt=fs.readFileSync("./a.txt");
iconv.decode(txt,"gbk")
request("http://jd.com")
.pipe(iconv.decodeStream('gbk'))
.collect(function(err, decodedBody) {
 fs.writeFile("./jd.txt",decodedBody);
});