声明Javascript变量时是否需要var ?(复制)

Possible Duplicate:
Javascript: is using 'var' to declare variables optional?

可能的重复:Javascript:使用“var”声明变量是可选的吗?

When creating variables in javascript is adding "var" before the variable name a must?

在javascript中创建变量是在变量名a之前添加“var”?

For example instead of

例如,而不是

var message = "Hello World!"

can I use

我可以用

message = "Hello World!"

吗?

I notice that scripts like Google Adsense don't use var

我注意到像谷歌Adsense这样的脚本不使用var

Example:

例子:

google_ad_width = 160;
google_ad_height = 600;
google_color_border = "000000";
google_color_bg = "ffffff";

6 个解决方案

#1

Without the var you create a global, and globals are a fantastic way to have different functions overwriting each others variables (i.e. they make code a pain to maintain).

没有了var，您就创建了一个全局变量，而globals是一种非常奇妙的方法，可以让不同的函数覆盖每个其他变量(也就是说，它们使代码难以维护)。

With the var, the scope of the variable is limited to the current function (and anything inside it — it is possible to nest functions).

对于var，变量的范围仅限于当前函数(以及其中的任何内容——可以嵌套函数)。

Google Adsense uses globals because it splits scripts into two distinct parts (one local and one remote). A cleaner approach would be to call a function defined in the remote script and pass the parameters as arguments instead of having it pick them up from the global scope.

谷歌Adsense使用全局变量，因为它将脚本分为两个不同的部分(一个本地部分和一个远程部分)。一种更简洁的方法是调用在远程脚本中定义的函数，并将参数作为参数传递，而不是让它从全局范围中获取参数。

#2

Yes, you should always use var.

是的，您应该始终使用var。

Not using var has two major drawbacks:

不使用var有两个主要缺点:

Accessing a variable within a function that is not defined within that function will cause the interpreter to look up the scope chain for a variable with that name until either it find one or it gets to the global object (accessible in browsers via window) where it will create a property. This global property is now available everywhere, potentially causing confusion and hard-to-detect bugs;
访问一个变量在一个函数内定义的函数,它不是将导致翻译查找一个变量的作用域链这个名字,直到它找到一个或到达全局对象(在浏览器访问通过窗口),它将创建一个属性。这个全局属性现在无处不在，可能导致混乱和难以检测的bug;
Accessing an undeclared variable will cause an error in ECMAScript 5 strict mode.
访问未声明的变量将导致ECMAScript 5严格模式中的错误。

Also, not using var for global variable is not exactly the same as using var: when using var, the property it creates on the global object has the internal DontDelete attribute, which is not the case without var:

此外，不将var用于全局变量并不完全等同于使用var:当使用var时，它在全局对象上创建的属性具有内部的DontDelete属性，没有var就不是这样:

// Next line works in any ECMAScript environment. In browsers, you can
// just use the window object.
var globalObj = (function() { return this; })();

var x = 1;
delete globalObj.x;
alert(x); // Alerts 1, x could not be deleted

y = 2;
delete globalObj.y;
alert(y); // Error, y is undefined

#3

From http://www.updrift.com/article/to-var-or-not-to-var-my-javascript

从http://www.updrift.com/article/to-var-or-not-to-var-my-javascript

For global variables, it doesn’t matter, but you may want to use it for consistency.
对于全局变量，这并不重要，但是您可能希望使用它来保持一致性。
Always try to use ‘var’ to declare variables in local functions. It makes sure you’re using a local copy of the variable instead of another variable of the same name in a different scope.
总是尝试使用' var '来声明本地函数中的变量。它确保您正在使用变量的本地副本，而不是在不同范围中使用同名的另一个变量。

For example, the two similar functions here have very different effects:

例如，这里两个相似的函数有非常不同的效果:

var myvar = 0;
function affectsGlobalVar(i){
   myvar = i;
}
function doesNotAffectGlobalVar(i){
   var myvar = i;
}

#4

Without var a variable is defined as global.

没有var，变量定义为全局变量。

#5

When adding Google AdSense to your page, you're linking a number of javascript files. The variables are defined in there.

在向页面添加谷歌AdSense时，您正在链接许多javascript文件。变量是在这里定义的。

http://pagead2.googlesyndication.com/pagead/show_ads.js should be one of the script references in your page.

http://pagead2.googlesyndication.com/pagead/show_ads.js应该是页面中的脚本引用之一。

#6

For the unenlightened like myself, JavaScript basically has two scopes for variables: global and local. Variables assigned outside of a function are global, and variables assigned inside of a function, using the var keyword, are local (not rocket surgery). However, if you leave the var keyword off, it assigns a global variable, regardless of where it’s declared.

对于像我这样的未开化的人，JavaScript基本上有两个变量作用域:全局变量和局部变量。函数外部分配的变量是全局的，函数内部分配的变量使用var关键字是局部的(不是rocket surgery)。但是，如果您不使用var关键字，它会分配一个全局变量，不管它声明在哪里。

From: http://opensoul.org/2008/9/4/the-importance-of-var-in-javascript

来自:http://opensoul.org/2008/9/4/the-importance-of-var-in-javascript

#1