blocks|key|926029|text|使用var是最干净的方式，但两者的工作原理都如下所示：https://developer.mozilla.org/en/JavaScript/Reference/Statements/for...in|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|926030|基本上，通过使用var，您可以确保创建一个新变量。否则，您可能会意外地使用先前定义的变量。|926031|entityMap|0|LINK|mutability|MUTABLE|url|https://developer.mozilla.org/en/JavaScript/Reference/Statements/for...in^0|2|3|R|21|0|0|8|3|0^^$0|@$1|2|3|4|5|6|7|P|8|@$9|Q|A|R|B|C]]|D|@$9|S|A|T|1|U]]|E|$]]|$1|F|3|G|5|6|7|V|8|@$9|W|A|X|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|Y|8|@]|D|@]|E|$]]]|I|$J|$5|K|L|M|E|$N|O]]]]

Using <code>var</code> is the cleanest way, but both work as described here: <a href="https://developer.mozilla.org/en/JavaScript/Reference/Statements/for...in" rel="noreferrer">https://developer.mozilla.org/en/JavaScript/Reference/Statements/for...in</a>

Basically, by using <code>var</code> you ensure that you create a new variable. Otherwise you might accidentally use a previously defined variable.

blocks|key|709844|text|使用使用var声明循环变量的方法。隐式声明的变量有一个不同的作用域，这可能不是您想要的。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|709845|entityMap^0|4|3|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|K|8|@]|D|@]|E|$]]]|G|$]]

Use the one where you declare the loop variable with <code>var</code>. Implicitly declared variables have a different scope that's probably not what you intended.

blocks|key|709870|text|出于性能方面的原因，我认为var很好。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|709871|Javascript不会查看整个全局作用域来查看x是否已经存在于其他地方。|709872|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

I think var is good for performance reasons. 

Javascript won't look through the whole global scope to see if x already exists somewhere else.

blocks|key|709945|text|从一般的观点来看，第一个版本的索引必须位于循环的作用域内，而另一个版本是调用循环的构造函数的作用域中的任何变量。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|709946|如果你要在for循环中使用循环的索引，并且在接下来的几行中其他人不需要这个，最好用"var“声明变量，这样你就可以确保"x”是用0初始化的循环的索引，而另一个变量，如果在这个上下文中有其他的"x“变量，这个变量将被循环的索引覆盖-+-那你就会有一些逻辑错误-。|709947|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

From a general point of view, first version will be for an index that must live within loop's scope, while the other one would be any variable in the scope where loop's constructor got invoked.

If you're going to use loop's index inside for loop and this won't be required by others in next lines, better declare the variable with "var" so you'll be sure "x" is for loop's index initialized with 0, while the other one, if other "x" variable is available in this context, this will get overwritten by loop's index - that's you'll have some logical errors -.

blocks|key|710006|text|使用var，它缩小了变量的作用域，否则变量将查找最近的闭包来搜索var语句。如果它不能找到一个var，那么它是全局的(如果你在严格模式下，using+strict，全局变量抛出一个错误)。这可能会导致如下问题。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|710007|function+f+(){
++++for+(i=0;+i<5;+i%2B%2B);
}
var+i+=+2;
f+();
alert+(i);+//i+==+5.+i+should+be+2|code-block|syntax|javascript|710008|如果在for循环中编写var+i，则警报将显示2。|710009|JavaScript+Scoping+and+Hoisting|710010|entityMap|0|LINK|mutability|MUTABLE|url|http://www.adequatelygood.com/JavaScript-Scoping-and-Hoisting.html^0|2|3|W|3|1B|3|1X|C|0|0|B|5|N|1|0|0|V|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]|$9|Z|A|10|B|C]|$9|11|A|12|B|C]|$9|13|A|14|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|15|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|16|8|@$9|17|A|18|B|C]|$9|19|A|1A|B|C]]|D|@]|E|$]]|$1|M|3|N|5|6|7|1B|8|@]|D|@$9|1C|A|1D|1|1E]]|E|$]]|$1|O|3|-4|5|6|7|1F|8|@]|D|@]|E|$]]]|P|$Q|$5|R|S|T|E|$U|V]]]]

Use <code>var</code>, it reduces the scope of the variable otherwise the variable looks up to the nearest closure searching for a <code>var</code> statement. If it cannot find a <code>var</code> then it is global (if you are in a strict mode, <code>using strict</code>, global variables throw an error). This can lead to problems like the following.

<pre><code>function f (){
 for (i=0; i&lt;5; i++);
}
var i = 2;
f ();
alert (i); //i == 5. i should be 2
</code></pre>

If you write <code>var i</code> in the for loop the alert shows <code>2</code>.

<a href="http://www.adequatelygood.com/JavaScript-Scoping-and-Hoisting.html" rel="noreferrer">JavaScript Scoping and Hoisting</a>

blocks|key|710115|text|第一个版本：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|710116|for+(var+x+in+set)+{
++++...
}|code-block|syntax|javascript|710117|声明一个名为x的局部变量。第二个版本：|offset|length|style|CODE|710118|for+(x+in+set)+{
++++...
}|710119|不会。|710120|如果x已经是一个局部变量(例如，您在当前作用域(即current函数)中较早的某个位置有一个var+x;或var+x+=+...;+)，那么它们将是等价的。如果x还不是一个局部变量，那么使用第二个变量将隐式声明一个全局变量x。考虑下面的代码：|710121|var+obj1+=+{hey:+10,+there:+15};
var+obj2+=+{heli:+99,+copter:+10};
function+loop1()+{
++++for+(x+in+obj1)+alert(x);
}
function+loop2()+{
++++for+(x+in+obj2)+{
++++++++loop1();+
++++++++alert(x);
++++}
}
loop2();|710122|您可能认为这会向hey、there、heli、hey、there、copter发出警报，但由于x是同一个，它将向hey、there、<代码>D18、<代码>D19、<代码>D20、<代码>D21发出警报。你不会想这样的！在for循环中使用var+x。|710123|最重要的是:如果for循环在全局作用域中(即不在函数中)，则局部作用域(如果使用var+x，则在中声明作用域x+)与全局作用域(如果使用不带x的x，则在中隐式声明作用域var)相同，因此两个版本将是相同的。|710124|entityMap^0|0|0|6|1|0|0|0|2|1|1A|6|1H|C|28|1|33|1|0|0|8|3|C|5|I|4|N|3|R|5|X|6|1B|1|1K|3|1O|5|33|3|3B|5|0|8|3|14|5|1I|1|1Y|1|20|1|0^^$0|@$1|2|3|4|5|6|7|10|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|11|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|12|8|@$I|13|J|14|K|L]]|9|@]|A|$]]|$1|M|3|N|5|D|7|15|8|@]|9|@]|A|$E|F]]|$1|O|3|P|5|6|7|16|8|@]|9|@]|A|$]]|$1|Q|3|R|5|6|7|17|8|@$I|18|J|19|K|L]|$I|1A|J|1B|K|L]|$I|1C|J|1D|K|L]|$I|1E|J|1F|K|L]|$I|1G|J|1H|K|L]]|9|@]|A|$]]|$1|S|3|T|5|D|7|1I|8|@]|9|@]|A|$E|F]]|$1|U|3|V|5|6|7|1J|8|@$I|1K|J|1L|K|L]|$I|1M|J|1N|K|L]|$I|1O|J|1P|K|L]|$I|1Q|J|1R|K|L]|$I|1S|J|1T|K|L]|$I|1U|J|1V|K|L]|$I|1W|J|1X|K|L]|$I|1Y|J|1Z|K|L]|$I|20|J|21|K|L]|$I|22|J|23|K|L]|$I|24|J|25|K|L]]|9|@]|A|$]]|$1|W|3|X|5|6|7|26|8|@$I|27|J|28|K|L]|$I|29|J|2A|K|L]|$I|2B|J|2C|K|L]|$I|2D|J|2E|K|L]|$I|2F|J|2G|K|L]]|9|@]|A|$]]|$1|Y|3|-4|5|6|7|2H|8|@]|9|@]|A|$]]]|Z|$]]

The first version:

<pre><code>for (var x in set) {
 ...
}
</code></pre>

declares a local variable called <code>x</code>. The second version:

<pre><code>for (x in set) {
 ...
}
</code></pre>

does not.

If <code>x</code> is already a local variable (i.e. you have a <code>var x;</code> or <code>var x = ...;</code> somewhere earlier in your current scope (i.e. the current function)) then they will be equivalent. If <code>x</code> is not already a local variable, then using the second will implicitly declare a global variable <code>x</code>. Consider this code:

<pre><code>var obj1 = {hey: 10, there: 15};
var obj2 = {heli: 99, copter: 10};
function loop1() {
 for (x in obj1) alert(x);
}
function loop2() {
 for (x in obj2) {
 loop1(); 
 alert(x);
 }
}
loop2();
</code></pre>

you might expect this to alert <code>hey</code>, <code>there</code>, <code>heli</code>, <code>hey</code>, <code>there</code>, <code>copter</code>, but since the <code>x</code> is one and the same it will alert <code>hey</code>, <code>there</code>, <code>there</code>, <code>hey</code>, <code>there</code>, <code>there</code>. You don't want that! Use <code>var x</code> in your <code>for</code> loops.

To top it all off: if the <code>for</code> loop is in the global scope (i.e. not in a function), then the local scope (the scope <code>x</code> is declared in if you use <code>var x</code>) is the same as the global scope (the scope <code>x</code> is implicitly declared in if you use <code>x</code> without a var), so the two versions will be identical.

blocks|key|926397|text|实际上，如果你不喜欢在for标题中声明，你可以这样做：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|926398|var+x;
for+(x+in+set)+{
++++...
}|code-block|syntax|javascript|926399|正如在对这个问题的其他回答中所提到的，根本不使用var会产生不必要的副作用，比如分配全局属性。|926400|entityMap^0|B|3|0|0|O|3|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|R|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|S|8|@$9|T|A|U|B|C]]|D|@]|E|$]]|$1|M|3|-4|5|6|7|V|8|@]|D|@]|E|$]]]|N|$]]

Actually, if you dislike declaration within <code>for</code> heading, you can do:

<pre><code>var x;
for (x in set) {
 ...
}
</code></pre>

As mentioned in other answers to this question, not using <code>var</code> at all produces unnecessary side-effects like assigning a global property.

blocks|key|926494|text|for(var+i+=+0;+...)|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|926495|是一种常见的模式，但它与|unstyled|926496|for(int+i;+...)|926497|在C%2B%2B中，变量的作用域不是for块。实际上，var被提升到封闭作用域(函数)的顶部，因此在for循环之前(在当前作用域/函数开始之后)和之后，都可以有效地使用本地i。|offset|length|style|CODE|926498|换句话说，做：|926499|(function(){+//beginning+of+your+current+scope;
+//...
+for(var+i+in+obj)+{+...+};
})();|926500|与以下内容相同：|926501|(function(){+//beginning+of+your+current+scope;
+var+i;
+//...
+for(i+in+obj)+{+...+};
})();|926502|ES6使用let关键字(而不是var)将作用域限制为for块。|926503|当然，您应该使用局部变量(用var或let或const+(在ES6中)声明的变量)，而不是隐式全局变量。|926504|如果您使用"use+strict";+(正如您应该做的那样)并且未声明i，则for(i=0;+...)或for(i+in+...)将失败。|926505|entityMap^0|0|0|0|E|3|N|3|1A|3|2A|1|0|0|0|0|0|5|3|F|3|0|E|3|I|3|M|5|0|5|D|Z|1|12|D|1G|D|0^^$0|@$1|2|3|4|5|6|7|14|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|15|8|@]|9|@]|A|$]]|$1|G|3|H|5|6|7|16|8|@]|9|@]|A|$B|C]]|$1|I|3|J|5|F|7|17|8|@$K|18|L|19|M|N]|$K|1A|L|1B|M|N]|$K|1C|L|1D|M|N]|$K|1E|L|1F|M|N]]|9|@]|A|$]]|$1|O|3|P|5|F|7|1G|8|@]|9|@]|A|$]]|$1|Q|3|R|5|6|7|1H|8|@]|9|@]|A|$B|C]]|$1|S|3|T|5|F|7|1I|8|@]|9|@]|A|$]]|$1|U|3|V|5|6|7|1J|8|@]|9|@]|A|$B|C]]|$1|W|3|X|5|F|7|1K|8|@$K|1L|L|1M|M|N]|$K|1N|L|1O|M|N]]|9|@]|A|$]]|$1|Y|3|Z|5|F|7|1P|8|@$K|1Q|L|1R|M|N]|$K|1S|L|1T|M|N]|$K|1U|L|1V|M|N]]|9|@]|A|$]]|$1|10|3|11|5|F|7|1W|8|@$K|1X|L|1Y|M|N]|$K|1Z|L|20|M|N]|$K|21|L|22|M|N]|$K|23|L|24|M|N]]|9|@]|A|$]]|$1|12|3|-4|5|F|7|25|8|@]|9|@]|A|$]]]|13|$]]

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

is a commonly seen pattern but it's different from 

<pre><code>for(int i; ...)
</code></pre>

in C++ in that that the variable isn't scoped to the <code>for</code> block. In fact, the <code>var</code> gets hoisted to the top of the enclosing scope (function) so a local <code>i</code> will be effectively available both before the <code>for</code> loop (after the beginning of the current scope/function) and after it.

In other words, doing:

<pre><code>(function(){ //beginning of your current scope;
 //...
 for(var i in obj) { ... };
})();
</code></pre>

is the same as:

<pre><code>(function(){ //beginning of your current scope;
 var i;
 //...
 for(i in obj) { ... };
})();
</code></pre>

ES6 has the <code>let</code> keyword (instead of <code>var</code>) to limit the scope to the for block.

Of course, you SHOULD be using local variables (ones declared with either <code>var</code> or <code>let</code> or <code>const</code> (in ES6)) rather than implicit globals.

<code>for(i=0; ...)</code> or <code>for(i in ...)</code> will fail if you use <code>"use strict";</code> (as you should) and <code>i</code> isn't declared.

blocks|key|710413|text|我总是使用ES2015中引入的块作用域let。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|710414|for+(let+x+in+set)+{
++++...
}|code-block|syntax|javascript|710415|Additional+reading+and+examples|710416|entityMap|0|LINK|mutability|MUTABLE|url|https://www.w3schools.com/js/js_let.asp^0|J|3|0|0|0|V|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@$9|V|A|W|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|X|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|Y|8|@]|D|@$9|Z|A|10|1|11]]|E|$]]|$1|M|3|-4|5|6|7|12|8|@]|D|@]|E|$]]]|N|$O|$5|P|Q|R|E|$S|T]]]]

I always use the block scoped <code>let</code> introduced in ES2015.
<pre><code>for (let x in set) {
 ...
}
</code></pre>
<a href="https://www.w3schools.com/js/js_let.asp" rel="nofollow noreferrer">Additional reading and examples</a>

What's the correct way to write a <code>for-in</code> loop in JavaScript? The browser doesn't issue a complaint about either of the two approaches I show here. First, there is this approach where the iteration variable <code>x</code> is explicitly declared:

<pre><code>for (var x in set) {
 ...
}
</code></pre>

And alternatively this approach which reads more naturally but doesn't seem correct to me:

<pre><code>for (x in set) {
 ...
}
</code></pre>

"var" or no "var" in JavaScript's "for-in" loop?

用JavaScript编写for-in循环的正确方法是什么？浏览器不会对我在这里展示的两种方法中的任何一种发出抱怨。首先，有一种方法是显式声明迭代变量x：for (var x in set) {    ...}或者，这种方法读起来更自然，但在我看来并不正确：for (x in set) {    ...}

问在JavaScript的"for- in“循环中有没有"var”？
EN

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问在JavaScript的"for- in“循环中有没有"var”？EN

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问在JavaScript的"for- in“循环中有没有"var”？
EN