var adList = new Array(); 
    adList[0]["img"] = 'http://www.gamer-source.com/image/media/screenshot/thumbnail/489.jpg';/* 
    adList[0]["h3"] = 'Product title2'; 
    adList[0]["button"]["link"] = '#link-url'; 
    adList[0]["button"]["text"] = 'Buy now'; 
    adList[0]["h3"] = 'asdfasdfasdf'; 
 
    adList[1]["img"] = 'http://www.gamer-source.com/image/media/screenshot/thumbnail/489.jpg'; 
    adList[1]["h3"] = 'Product title2'; 
    adList[1]["button"]["link"] = '#link-url'; 
    adList[1]["button"]["text"] = 'Buy now'; 
    adList[1]["h3"] = 'asdfasdfasdf'; 

我遇到错误 adList[0] is undefined，我不能像这样定义数组吗？在为它分配变量之前，是否必须将 adList[0] 指定为数组？

请您参考如下方法:

问题是您试图引用尚未分配值的 adList[0]。因此，当您尝试访问 adList[0]['img'] 的值时，会发生这种情况:

adList           -> Array 
adList[0]        -> undefined 
adList[0]["img"] -> Error, attempting to access property of undefined. 

尝试使用数组和对象文字符号来定义它。

adList = [{ 
    img: 'http://www.gamer-source.com/image/media/screenshot/thumbnail/489.jpg', 
    h3: 'Product title 1', 
    button: { 
        text: 'Buy now', 
        url: '#link-url' 
    } 
}, 
{ 
    img: 'http://www.gamer-source.com/image/media/screenshot/thumbnail/489.jpg', 
    h3: 'Product title 2', 
    button: { 
        text: 'Buy now', 
        url: '#link-url' 
    } 
}]; 

或者您可以简单地分别定义 adList[0] 和 adList[1] 并使用您的旧代码:

var adList = new Array(); 
adList[0] = {}; 
adList[1] = {}; 
adList[0]["img"] = 'http://www.gamer-source.com/image/media/screenshot/thumbnail/489.jpg'; 
... etc etc