FOR循环和字符串与JavaScript连接给我一个未定义的值 [英] FOR loop and string concatenating with JavaScript gives me an undefined value

问题描述

我有数组

var data = [name, address, city, country];

循环

var columns;
for (var i = 0; i < data.length; i++) {
    columns += "data[" + i + "], ";
}
columns = columns.slice(0, -2);
alert(columns);

警告信息显示

undefineddata[0], data[1], data[2], data[3]

我在这里做错了什么？我想删除未定义的...

What am I doing wrong here? I want to remove the undefined...

推荐答案

你需要用空字符串启动，像这样：

You need to start with an empty string, like this:

var columns = "";

现在你拥有的基本相当于：

Right now what you have is basically equivalent to:

var columns = undefined;

当连接到字符串时，会给你undefined。

Which when concatenated to a string, gives you "undefined".

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