了解 javascript 全局命名空间和闭包

2022-01-14 00:00:00 namespaces global-variables closures javascript

我正在努力提高我对 javascript 中全局命名空间的理解，我对以下几点感到好奇:

是否有一个GOD"(即父)对象，所有对象(因为除了原始对象之外的所有事物都是对象)都可以回答，如果有，该对象是否会是窗口"?
李>
为什么在全局级别上有变量/函数是个坏主意?

如果在全局范围内拥有变量/函数真的是个坏主意，那么闭包是避免这种情况的最佳方法吗?示例:

函数父(){var x = 'some value';//这个 var 将被认为对所有子函数都是全局的，但不在真正的全局命名空间中函数 child1(){x.someMethod()}功能孩子2(){x*某事；}函数 child3(){x+=某事；孩子2()孩子1()}孩子3()}父母()

解决方案

<块引用>
是否有一个神(即父母)对象?
是的. 从技术上讲，它是所有这些原语的成员的全局对象；只是碰巧在浏览器中，window 对象是全局对象.
```
<代码>>window.String === 字符串；真的
```
<块引用>
为什么在全局级别上有变量/函数是个坏主意?
因为如果您要添加大量第 3 方库/脚本，它们都共享同一个全局对象，因此可能会发生名称冲突.这是所有使用 $ 作为别名的库(jQuery、Prototype 等)的现实问题.
<块引用>
如果在全局范围内拥有变量/函数真的是个坏主意，那么闭包是避免这种情况的最佳方法吗?
x 不应被视为全局.它是通过在 parent() 函数中声明子函数而形成的闭包的一部分.您的代码段的 problem 部分是 parent() 是全局的；如果其他代码重新声明 parent() 会发生什么?这样会更好:
```
(function () {函数父(){var x = '某个值';函数 child1(){x.someMethod()}功能孩子2(){x*某事；}函数 child3(){x+=某事；孩子2()孩子1()}孩子3()}父母()}());
```
x 可以在子函数中访问这一事实还不错；你应该自己编写这些函数，所以你应该意识到x的存在.请记住，如果您在这些子函数中使用 var 重新声明 x，则不会影响 parent 中的 x().

I'm trying to improve my understanding of the global namespace in javascript and I'm curious about a few things:

is there a "GOD" (i.e. a parent) object that all objects (since all things except primitives are objects) to answer to and if so would that object be "window" ?
why is it bad idea to have vars/functions on a global level?

if it is really a bad idea to have vars/functions in global scope then would closures be the best way to avoid this? example:

function parent(){
    var x = 'some value';//this var would be considered global to all children functions but not in the true global namespace
    function child1(){
        x.someMethod()
    } 
    function child2(){
        x*something;
    }
    function child3(){
        x+=something;
        child2()
        child1()
    }
    child3()
}
parent()

解决方案

Is there a god (i.e. a parent) object?

Yes. More technically, it's the global object that all these primitives are members of; it just happens that in the browser, the window object is the global object.
```
> window.String === String;
true
```
Why is it bad idea to have vars/functions on a global level?

Because if you're adding lots of 3rd party libraries/ scripts, they all share the same global object, there's the chance of name collisions. This is a real life problem with all the libraries which use $ as an alias (jQuery, Prototype and more).
If it is really a bad idea to have vars/functions in global scope then would closures be the best way to avoid this?

x shouldn't be considered global. It's part of the closure formed by declaring the child functions inside the parent() function. The problem part of your snippet is that parent() is global; what happens if some other code re-declared parent()? This would be better:
```
(function () {

function parent(){
    var x = 'some value';
    function child1(){
        x.someMethod()
    } 
    function child2(){
        x*something;
    }
    function child3(){
        x+=something;
        child2()
        child1()
    }
    child3()
}
parent()

}());
```
The fact x is accessible within the child functions isn't bad; you should have written those functions yourself, so you should be aware of the existence of x. Bear in mind that if you re-declare x within those child functions with var, you won't affect the x in parent().

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