未定义 - 烦人的 JavaScript 错误

Question

I have the following code and cannot understand why I am getting an 'undefined' before my object properties are listed..? Am I doing something obviously wrong?

As you can tell I am a newbie to JavaScript, any help is majorly appreciated!

 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; declares the variable and (implicitly) gives it an initial value of undefined .

player_profile += some_string then appends a string to it. This converts undefined to a string, the result of which is "undefined" .

If you want the initial value to be an empty string, then say so explicitly:

let player_profile = "";

Answer 2

Because you've not initialized your 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);