json对象中的未定义项

Question

我试图在for每个JSON列表中输出所有值,每次弹出这个神秘的"未定义"变量.看一看:

HTML:

<span id="1">

</span>

JavaScript的:

var lel =
    {
        1: {
            "some": "json",
            "list": "that",
            "says": "undefined",
        },
    }

var s;

for(i in lel[1]) {
     s+= '<B>' + i + '</B>' + ": " + lel[1][i] + "<br />"; 
}

document.getElementById('1').innerHTML = s;

结果(跨度)

undefined**some**: json
**list**: that
**says**: undefined

"未定义"的东西来自哪里？

Answer 1

问题是s最初是undefined.

改变var s;以var s = '';