未定义 - 烦人的 JavaScript 错误

Question

我有以下代码，但无法理解为什么在列出我的对象属性之前会得到“未定义”？ 我做错了什么吗？

正如您所知道的，我是 JavaScript 的新手，非常感谢任何帮助！

 let player_profile; const players = [ { name: "George Ford", age: 22, position: "Back" }, { name: "Ben Youngs", age: 28, position: "Forward" } ]; for (let i = 0; i < players.length; i++) { player_profile += '<h2>Name: ' + players[i].name + '</h2>'; player_profile += '<p>Age: ' + players[i].age + '</p>'; } document.write(player_profile);

Answer 1

let player_profile; 声明变量并（隐式）赋予它一个初始值undefined 。

player_profile += some_string然后附加一个字符串。 这会将undefined转换为字符串，其结果是"undefined" 。

如果你希望初始值是一个空字符串，那么明确地说：

let player_profile = "";

Answer 2

因为你还没有初始化你的 player_profile：

let player_profile = "";

const players = [
    {
        name: "George Ford",
        age: 22,
        position: "Back" 
    },
    {
        name: "Ben Youngs",
        age: 28,
        position: "Forward" 
    }
];

for (let i = 0; i < players.length; i++) {
  player_profile += '<h2>Name: ' + players[i].name + '</h2>';
  player_profile += '<p>Age: ' + players[i].age + '</p>';
}

document.write(player_profile);