什么是“与...”用JavaScript做什么？ [英] What does "with" do in JavaScript?

问题描述

我看到的JavaScript代码以开头，带。这有点令人困惑。它做了什么以及如何正确使用？

I saw JavaScript code which begins with with. That's a bit confusing. What does it do and how can it be used correctly?

with (sObj) return options[selectedIndex].value;

推荐答案

它增加了包含在block：

It adds to the scope of the statements contained in the block:

return sObj.options[selectedIndex].value;

可以成为：

with (sObj)
    return options[selectedIndex].value;

在你的情况下，它并没有做很多......但请考虑以下因素： / p>

In your case, it doens't do a whole lot...but consider the following:

var a, x, y;
var r = 10;
a = Math.PI * r * r;
x = r * Math.cos(PI);
y = r * Math.sin(PI /2);

成为：

var a, x, y;
var r = 10;
with (Math) {
  a = PI * r * r;
  x = r * cos(PI);
  y = r * sin(PI / 2);
}

...保存几次按键。 Mozilla文档实际上可以更详细地解释一些事情（以及使用它的优点和缺点）：

...saves a couple of keystrokes. The Mozilla documentation actually does a pretty good job of explaining things in a little more detail (along with pros and cons of using it):

with - Mozilla Developer Center

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