否自调用功能? ！function($){...}(window.jQuery); [英] negating self invoking function? !function ($) { ... }(window.jQuery);

问题描述

可能重复:
在功能之前，感叹号有什么作用?

Possible Duplicate:
What does the exclamation mark do before the function?

我在浏览Twitter Bootstrap JavaScript代码时，发现它们的所有插件都包装在否定自调用功能中.

I was looking through the Twitter Bootstrap JavaScript code and I noticed all their plugins are wrapped in negating self invoking functions.

我知道function ($) { ... }(window.jQuery);立即调用该函数.

I am aware that function ($) { ... }(window.jQuery); invokes the function immediately.

但是!的作用是什么?

推荐答案

1)，只要您这样做:

1) if you just do:

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

您将得到一个错误.
JS不允许您触发函数声明(由于可变提升"的概念，从技术上讲这不是正确的，但对于许多人来说更容易思考).
如果您使用声明性功能:

You will get an error.
JS will not allow you to fire off a function declaration (because of a concept of "variable hoisting", which isn't technically correct, but is much easier for a lot of people to think about).
If you use declarative functions:

function myFunc () { /* ... */ }

在创建范围变量之前，JS引擎将在您所在的范围内创建它们-即使var都在顶部，而函数都在底部.

The JS engine will create them in the scope you're in, before creating the variables of the scope -- even if the vars are all on the top and the functions are all on the bottom.

所以当你写的时候:

function myFunc () { /* ... */ }();

那里有一个错误，因为在当前作用域中甚至还不存在的var可能会在内部调用，这肯定会导致崩溃.

There's an error there, because the vars which don't even exist yet, in the current scope might be called inside, which is a guaranteed crash.

为进一步说明这一点，如果您不给函数声明起一个名字:

To further that, if you don't give your function-declaration a name:

function () { /* ... */ }

...然后那里有一个错误，无论如何，因为您还不能调用该函数，但是您无法获取它，因为它没有名称，也没有分配它到变量.

...then there's an error there, regardless, because you can't call the function yet, but you have no way of getting at it, because it doesn't have a name, and you didn't assign it to a variable.

()或+或!之类的运算符-会迫使引擎评估接下来发生的事情的任何事物，都将允许您触发以下函数:

Operators like () or + or ! -- anything which will force the engine to evaluate whatever comes next, will allow you to fire the function like:

+function () { /* ... */ }();

只要没有变量在监听return语句，就可以了.

As long as there is no variable listening for a return statement, you're fine.

2)另外，对于!function () { /*...*/ }();，如果该函数没有自然的return语句，则它将返回undefined.
以这种方式强制时，undefined的反面是true ...

2) Additionally, in the case of !function () { /*...*/ }();, if that function doesn't have a return statement naturally, then it will return undefined.
The opposite of undefined is true when coerced in that way...

...因此，如果您真的想要，您可以执行以下操作:

...so if you really, REALLY wanted, you could do something like:

var success = !function () { /* ... */ }();
if (success) { doStuff(); }

这篇关于否自调用功能? ！function($){...}(window.jQuery);的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！