使用node+express+http-proxy-middleware代理轉(zhuǎn)發(fā)解決跨域，獲取數(shù)據(jù)失敗

如題，使用express+http-proxy-middleware代理拿不到數(shù)據(jù)

這是app.js的內(nèi)容

var express = require('express');
var path = require('path');
var favicon = require('serve-favicon');
var logger = require('morgan');
var cookieParser = require('cookie-parser');
var bodyParser = require('body-parser');
// 代理插件
var proxy = require('http-proxy-middleware');
// 跨域插件
var cors = require('cors');

var index = require('./routes/index');
var users = require('./routes/users');

var app = express();
app.use(cors());

// view engine setup
app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'jade');

// uncomment after placing your favicon in /public
// app.use(favicon(path.join(__dirname, 'public', 'favicon.ico')));
app.use(logger('dev'));
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({
    extended: false
}));
app.use(cookieParser());
app.use(express.static(path.join(__dirname, 'public')));

app.use('/', index);
app.use('/users', users);


// catch 404 and forward to error handler
app.use(function(req, res, next) {
    var err = new Error('Not Found');
    err.status = 404;
    next(err);
});

// error handler
app.use(function(err, req, res, next) {
    // set locals, only providing error in development
    res.locals.message = err.message;
    res.locals.error = req
        .app
        .get('env') === 'development' ?
        err : {};

    // render the error page
    res.status(err.status || 500);
    res.render('error');
});

// 設(shè)置代理
app.use('/api', proxy({
    target: 'https://api.douban.com',
    changeOrigin: true
}));
app.listen(4000, function() {
    console.log('http://localhost:4000')
});


module.exports = app;

抓取的是豆瓣音樂的api，然后客戶端的代碼如下

function ajax(url) {
            var xml = new XMLHttpRequest();

            xml.open('GET', url, true);

            xml.onreadystatechange = function() {
                if (xml.readyState == 4 && xml.status == 200) {
                    console.log(JSON.parse(xml.responseText));
                }
            }

            xml.send();
        }

        ajax('http://localhost:4000/v2/music/search?q=周杰倫')

然而報(bào)錯(cuò)，拿不到數(shù)據(jù)，跨域確實(shí)沒有了，但是卻拿不到數(shù)據(jù)。

api接口地址是豆瓣音樂api，是可以正常打開的。不知道是不是我哪里寫錯(cuò)了，大佬們幫看看哇，感激不盡。