Google 分析跟踪代码使用的技术是什么? [英] What's the technique that the Google analytics tracking code uses?

问题描述

Google Analytics 跟踪代码如下所示:

The Google Analytics tracking code looks like this:

(function() {
code
  })();

他们对这些括号使用的技术是什么 - (stuff)() - 被称为?它有什么作用?

What's the technique they are using with those brackets - (stuff)() - called? What does it do?

我将 Google Analytics 代码放在页面上的关闭 head 标记之前，然后像这样在它周围放置一个 if 语句(我在上面包含了一个 Jquery cookie 插件):

I put the Google Analytics code before the closing head tag on my page and then put an if statement around it like this (I include a Jquery cookie plugin further up):

<script type="application/javascript">
if ($.cookie('allowcookies') == 'yes') {
analytics code
}
</script>

直到我在我的代码中使用相同的技术它才运行:

It didn't run until I used the same technique around my code:

(function() {if ($.cookie('allowcookies') == 'yes') {

analytics code
}
})();

为什么在我这样做之前它没有运行?为什么要追?

Why did it not run before I did that? Why did it run after?

推荐答案

(function() {
   /* code */
}()); 

它通常被称为«自执行匿名函数 (¹)»(o «立即函数调用»)，其主要用途是避免在全局(或外部)范围内创建变量.

It's commonly known as «self executed anonymous function (¹)» (o «immediate function invocation») and its main use is to avoid the creation of variables into the global (or in the outer) scope.

当您想创建一个只执行一次的函数时，它也可以用作快捷方式，而无需先用自己的标识符定义函数，然后很快进行函数调用.

It's also used as shortcut when you want to create a function to execute just once, without the need to first define the function with its own identifier and then soon make the function call.

它可能最终在作用域内使用，然后如果外部上下文(或其他引用)通过参数传递绑定，则它可能会创建一个闭包，例如

It may be eventually used inside a scope and then it may create a closure if the outer context (or other references) are binded through parameters passing, e.g.

/* outer scope */  
(function(outerscope) {

   element.onsomeevent = function() {
       /* do something with outerscope */
   };

}(this));

我用这个表达式做的另一个实际用途是当我需要创建一个函数时，当使用 new 关键字(而不是显式调用某些 newcode>init 方法).

Another practical use I make with this expression is when I need to create a function to be soon executed inside a constructor function when it is called with new keyword (instead of an explicit call to some init method).

(¹) —正如 Nicholas Zakas (O'Reilly, ISBN 978-1-449-32768-2) 页面 44 所著的Mantainable Javascript"一书所述，建议的表达式为 (function() {}())，带有嵌套括号(即使 (function() {})() 仍然可以工作)

(¹) — as stated on book "Mantainable Javascript" by Nicholas Zakas (O'Reilly, ISBN 978-1-449-32768-2) page 44, the suggested expression is (function() {}()), with nested parens (even if (function() {})() will work anyway)

[...]为了让函数立即调用更明显，在函数[...]

[...]To make it obvious that immediate function invocation is taking place, put paretheses around the function[...]

另见立即函数调用语法

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