blocks|key|1102777|text|如果函数中没有var声明，则对i的引用将指向全局对象的i属性。如果for循环主体中的代码调用也修改全局i的代码(例如，在被调用的方法中)，则可能会发生各种不可预测的行为。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1102778|注意，在var+i循环初始化中声明for|1102779|for+(var+i+=+...)|code-block|syntax|javascript|1102780|等于在var+i;循环之前声明for：|1102781|var+i;
for+(i+=+...)|1102782|特别是，i的声明将是吊起到封闭作用域的顶部。|1102783|编辑:如果启用严格模式，则必须声明循环变量(实际上是所有变量)。引用没有用var语句声明的变量将导致抛出ReferenceError+(而不是导致全局变量的存在)。|1102784|entityMap|0|LINK|mutability|MUTABLE|url|https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/var#var_hoisting|1|https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode^0|7|3|F|1|R|1|X|3|1F|1|0|4|5|H|3|0|0|3|6|F|3|0|0|4|1|A|2|0|0|11|3|1G|E|7|4|1|0^^$0|@$1|2|3|4|5|6|7|14|8|@$9|15|A|16|B|C]|$9|17|A|18|B|C]|$9|19|A|1A|B|C]|$9|1B|A|1C|B|C]|$9|1D|A|1E|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|1F|8|@$9|1G|A|1H|B|C]|$9|1I|A|1J|B|C]]|D|@]|E|$]]|$1|H|3|I|5|J|7|1K|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|1L|8|@$9|1M|A|1N|B|C]|$9|1O|A|1P|B|C]]|D|@]|E|$]]|$1|O|3|P|5|J|7|1Q|8|@]|D|@]|E|$K|L]]|$1|Q|3|R|5|6|7|1R|8|@$9|1S|A|1T|B|C]]|D|@$9|1U|A|1V|1|1W]]|E|$]]|$1|S|3|T|5|6|7|1X|8|@$9|1Y|A|1Z|B|C]|$9|20|A|21|B|C]]|D|@$9|22|A|23|1|24]]|E|$]]|$1|U|3|-4|5|6|7|25|8|@]|D|@]|E|$]]]|V|$W|$5|X|Y|Z|E|$10|11]]|12|$5|X|Y|Z|E|$10|13]]]]

Without a <code>var</code> declaration somewhere in a function, references to <code>i</code> will be to the <code>i</code> property of the global object. This risks all sorts of unpredictable behavior if code in the body of the <code>for</code> loop invokes code (say, inside a called method) that also modifies the global <code>i</code>.

Note that declaring <code>var i</code> in the <code>for</code> loop initialization

<pre><code>for (var i = ...)
</code></pre>

is equivalent to declaring <code>var i;</code> before the <code>for</code> loop:

<pre><code>var i;
for (i = ...)
</code></pre>

In particular, the declaration of <code>i</code> will be <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/var#var_hoisting" rel="nofollow">hoisted</a> to the top of the enclosing scope.

EDIT: If you enable <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode" rel="nofollow">strict mode</a>, then you must declare your loop variables (all variables, actually). Referencing a variable that has not been declared with a <code>var</code> statement will result in a <code>ReferenceError</code> being thrown (rather than resulting in a global variable coming into existence).

blocks|key|2399564|text|如果您说开始使用"i“而没有先说"var”，那么应该在for循环之前声明变量。比如：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2399565|​|2399566|var+i;
for+(i+=+0;+i+<+x;+i%2B%2B)+{
++doSomething();
}

==OR==
++
for(var+i+=+0;+i+<+x;+i%2B%2B){
++doSomething();++
}|code-block|syntax|javascript|2399567|2399568|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|M|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|N|8|@]|9|@]|A|$G|H]]|$1|I|3|C|5|6|7|O|8|@]|9|@]|A|$]]|$1|J|3|-4|5|6|7|P|8|@]|9|@]|A|$]]]|K|$]]

If you say start using "i" without saying "var" first, you should have declared the variable before your for loop.
Like:

<div class="snippet" data-lang="js" data-hide="false">
<div class="snippet-code">
<pre class="snippet-code-js lang-js prettyprint-override"><code>var i;
for (i = 0; i &lt; x; i++) {
 doSomething();
}

==OR==
 
for(var i = 0; i &lt; x; i++){
 doSomething(); 
}</code></pre>
</div>
</div>

blocks|key|2399610|text|JavaScript在功能范围上是限定的。至少目前，该语言没有块级变量。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2399611|在编写for+(var+i=0;+...+);时，它与|offset|length|style|CODE|2399612|var+i;
for+(i=0;+...+);|code-block|syntax|javascript|2399613|在没有"use+strict";的情况下，变量声明将是hoisted。|2399614|仅for+(i=0;+...+);就意味着i属于全局(最顶层)对象(在浏览器中它是window)|2399615|entityMap|0|LINK|mutability|MUTABLE|url|https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode|1|https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/var^0|0|3|K|0|0|3|D|R|7|3|D|0|R|7|1|0|1|G|L|1|15|6|0^^$0|@$1|2|3|4|5|6|7|10|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|11|8|@$D|12|E|13|F|G]]|9|@]|A|$]]|$1|H|3|I|5|J|7|14|8|@]|9|@]|A|$K|L]]|$1|M|3|N|5|6|7|15|8|@$D|16|E|17|F|G]|$D|18|E|19|F|G]]|9|@$D|1A|E|1B|1|1C]|$D|1D|E|1E|1|1F]]|A|$]]|$1|O|3|P|5|6|7|1G|8|@$D|1H|E|1I|F|G]|$D|1J|E|1K|F|G]|$D|1L|E|1M|F|G]]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|1N|8|@]|9|@]|A|$]]]|R|$S|$5|T|U|V|A|$W|X]]|Y|$5|T|U|V|A|$W|Z]]]]

JavaScript is functionally scoped. At least for now, the language does not have block level variables.

When you write <code>for (var i=0; ... );</code> it is the same as 

<pre><code>var i;
for (i=0; ... );
</code></pre>

In the absence of <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode" rel="nofollow"><code>"use strict";</code></a> variable declaration will be <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/var" rel="nofollow"><code>hoisted</code></a>. 

<code>for (i=0; ... );</code> alone implies that <code>i</code> belongs to the global (top most) object (in browsers it's <code>window</code>)

blocks|key|1102925|text|简介：|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|entityRanges|data|1102926|for语句创建一个由三个可选表达式组成的循环。|blockquote|1102927|资料来源:+Mozilla+JavaScript文档|1102928|背景：|1102929|您指的是初始化，它是表达式或变量声明。它几乎总是用于初始化一个计数器变量，如您所示，它允许我们迭代一个集合。|1102930|此表达式可以选择使用var关键字声明新变量。这些变量不是循环的本地变量，即它们位于for循环所在的相同范围内。此表达式的结果将被丢弃。|1102931|在JavaScript中，变量可以保存不同的数据类型，而在计数器变量的情况下，JavaScript将变量视为数字。|1102932|答案：|1102933|您可以选择声明一个新变量(或者完全不声明)的原因是JavaScript编程语言的特性。您已经发现了语言中处理变量和作用域的一个重要方面。|1102934|“变还是不变”|1102935|请参阅另一篇关于使用var或根本不使用var的帖子，以了解更多.|1102936|var关键字的功能是什么?何时使用它(或省略它)？|1102937|entityMap|0|LINK|mutability|MUTABLE|url|https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for|1|https://stackoverflow.com/questions/1470488/what-is-the-function-of-the-var-keyword-and-when-to-use-it-or-omit-it^0|0|3|0|0|0|Q|0|0|0|3|0|0|0|0|0|3|0|0|0|0|0|P|1|0^^$0|@$1|2|3|4|5|6|7|1C|8|@$9|1D|A|1E|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|1F|8|@]|D|@]|E|$]]|$1|I|3|J|5|6|7|1G|8|@]|D|@$9|1H|A|1I|1|1J]]|E|$]]|$1|K|3|L|5|6|7|1K|8|@$9|1L|A|1M|B|C]]|D|@]|E|$]]|$1|M|3|N|5|6|7|1N|8|@]|D|@]|E|$]]|$1|O|3|P|5|H|7|1O|8|@]|D|@]|E|$]]|$1|Q|3|R|5|6|7|1P|8|@]|D|@]|E|$]]|$1|S|3|T|5|6|7|1Q|8|@$9|1R|A|1S|B|C]]|D|@]|E|$]]|$1|U|3|V|5|6|7|1T|8|@]|D|@]|E|$]]|$1|W|3|X|5|6|7|1U|8|@]|D|@]|E|$]]|$1|Y|3|Z|5|6|7|1V|8|@]|D|@]|E|$]]|$1|10|3|11|5|6|7|1W|8|@]|D|@$9|1X|A|1Y|1|1Z]]|E|$]]|$1|12|3|-4|5|6|7|20|8|@]|D|@]|E|$]]]|13|$14|$5|15|16|17|E|$18|19]]|1A|$5|15|16|17|E|$18|1B]]]]

Intro:

<blockquote>
 The for statement creates a loop that consists of three optional
 expressions
</blockquote>

<a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for" rel="nofollow noreferrer">Source: Mozilla JavaScript Docs</a>

Background:

You are referring to initialization, which is an expression or variable declaration. It's almost always used to initialize a counter variable that allows us to iterate through a collection as you've shown.

<blockquote>
 This expression may optionally declare new variables with the var
 keyword. These variables are not local to the loop, i.e. they are in
 the same scope the for loop is in. The result of this expression is
 discarded.
</blockquote>

In JavaScript, variables can hold different data types, and in the case of a counter variable, JavaScript treats the variable as a number.

Answer:

The reason one can optionally declare a new variable (or not at all, as you've highlighted) is due to the nature of the JavaScript programming language. You've hit upon an important aspect of the language that deals with variables and scope.

"To var or not to var"

Please see this other post about using var or not using it at all to understand more...

<a href="https://stackoverflow.com/questions/1470488/what-is-the-function-of-the-var-keyword-and-when-to-use-it-or-omit-it">What is the function of the var keyword and when to use it (or omit it)?</a>

I've seen Javascript code that has used two different ways of defining a for loop.

<pre><code>for (var i=0;i &lt; x.length; i++)
</code></pre>

But it's also been 

<pre><code>for (i=0; i &lt; x.length; i++)
</code></pre>

The same thing has happened with <code>for-in</code> loops

<pre><code>for (var i in x)
</code></pre>

and 

<pre><code>for (i in x)
</code></pre>

Is there any difference between declaring <code>i</code> as a <code>var</code> and just saying <code>i</code>? Are there advantages of doing one over the other? Is one these right way to do this? From what I can tell, they both act the same, but there has to be some difference.

Note: I'm not asking about the difference between <code>for-in</code> and <code>for (i=0)</code>

Using var when declaring i in a for loop

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我看到Javascript代码使用了两种不同的方法来定义for循环。for (var i=0;i < x.length; i++)但它也是for (i=0; i < x.length; i++)同样的事情也发生在for-in循环中。for (var i in x)和for (i in x)将i声明为var与只说i有什么...

问在for循环中声明i时使用var
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问在for循环中声明i时使用varEN