比如我想读取的数据的网址是：https://api.douban.com/v2/book/1220562

里面的数据：{ “id”:“1220562”, “alt”:“https://book.douban.com/book/1220562”, “rating”:{“max”:10, “average”:“7.0”, “numRaters”:282, “min”:0}, “author”:[{“name”:“片山恭一”}, {“name”:“豫人”}], “alt_title”:"", “image”:“https://img3.doubanio.com/spic/s1747553.jpg”, “title”:“满月之夜白鲸现”}

我想要"average":“7.0” 这个7.0这个数据。

代码如下：

var fullUrl = “https://api.douban.com/v2/book/1220562”; xmlhttp=new XMLHttpRequest(); xmlhttp{ method: ‘GET’, url: fullUrl, headers: { ‘User-agent’: ‘Mozilla/4.0 (compatible) Greasemonkey’, ‘Accept’: ‘application/atom+xml,application/xml,text/xml’, }, onload: function(responseDetails) {

    eval( responseDetails.responseText );
    这里怎么写？？？
}

};