对象的Javascript数组未定义

马蒂金斯

我试图用javascript制作对象数组，但是当我尝试将数组打印到控制台（使用chrome）时，我得到的都是不确定的，除非我在推送后立即打印数组。

 var subitems = [];
 for (var i = 1; i < 1000; i = i + 50) {
       $.getJSON('http://gdata.youtube.com/feeds/api/users/' + username + '/subscriptions?max-results=50&start-index=' + i + '&alt=json', function (data) {
           var output = "";
           for (var x in data.feed.entry) {
               var subtitle = data.feed.entry[x].yt$username.$t;
               var subchannel = data.feed.entry[x].yt$channelId.$t;
               var subphoto = data.feed.entry[x].media$thumbnail.url;
               subitems.push({
                   subtitle: subtitle,
                   subchannel: subchannel,
                   subphoto: subphoto
               });
               console.log(subitems[0]); //Prints Fine
               output += '<div class="subtitle"><img src="' + subphoto + '"> <a href="http://www.youtube.com/channel/' + subchannel + '">' + propercase(subtitle) + '</a></div>';
           }
           maincontent.innerHTML = maincontent.innerHTML + output;
       });
 }
 console.log(subitems[0]); //This is undefined

萨尔曼A

这是这样做的“简便”方法：计算XHR请求并在所有请求返回后触发完成函数：

var xhrCount = 0;
for (var i = 1; i < 1000; i = i + 50) {
    xhrCount++; // increment before request
    $.getJSON('http://gdata.youtube.com/feeds/api/users/' + username + '/subscriptions?max-results=50&start-index=' + i + '&alt=json', function (data) {
            // process response      
            // decrement after request is processed
            xhrCount--;
            if (xhrCount === 0) {
                // at this point all requests have returned  
                console.log(subitems);
            }
        }
    }
}

更新：一种更好的方法是利用jQuery.Deferred()对象。这里设置了一个快速演示，需要一些修复。总体思路是跟踪您提出的所有AJAX请求，将其传递给，deferred.when()并使用分配完成的功能deferred.done()。代码（为便于阅读而精简）：

var subitems = [];
var deferreds = [];
for (var i = 1; i < 1000; i = i + 50) {
    var deferred = $.getJSON('https://gdata.youtube.com/feeds/api/users/warnerbrospictures/uploads?max-results=50&start-index=' + i + '&alt=json', function (data) {
        for (var j = 0; j < data.feed.entry.length; j++) {
            subitems.push({
                title: data.feed.entry[j].title.$t
            });
        }
    });
    deferreds.push(deferred);
}
$.when.apply($, deferreds).done(function () {
    console.clear();
    console.log(subitems);
    for (var i = 0; i < subitems.length; i++) {
        console.log(subitems[i].title);
    }
});