blocks|key|1297849|text|不，它们不是等价的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1297850|在myObj+=+1;中，您使用的是全局变量。|offset|length|style|CODE|1297851|后一个声明创建一个您正在使用的作用域的局部变量。|1297852|尝试使用以下代码来了解其中的差异：|1297853|external+=+5;
function+firsttry()+{
++var+external+=+6;
++alert("first+Try:+"+%2B+external);
}

function+secondtry()+{
++external+=+7;
++alert("second+Try:+"+%2B+external);
}

alert(external);+//+Prints+5
firsttry();+//+Prints+6
alert(external);+//+Prints+5
secondtry();+//+Prints+7
alert(external);+//+Prints+7|code-block|syntax|javascript|1297854|第二个函数改变全局变量"external“的值，但第一个函数不会。|1297855|entityMap^0|0|1|A|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|V|8|@$D|W|E|X|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|Y|8|@]|9|@]|A|$]]|$1|J|3|K|5|6|7|Z|8|@]|9|@]|A|$]]|$1|L|3|M|5|N|7|10|8|@]|9|@]|A|$O|P]]|$1|Q|3|R|5|6|7|11|8|@]|9|@]|A|$]]|$1|S|3|-4|5|6|7|12|8|@]|9|@]|A|$]]]|T|$]]

Nope, they are not equivalent.

With <code>myObj = 1;</code> you are using a global variable.

The latter declaration create a variable local to the scope you are using.

Try the following code to understand the differences:

<pre><code>external = 5;
function firsttry() {
 var external = 6;
 alert("first Try: " + external);
}

function secondtry() {
 external = 7;
 alert("second Try: " + external);
}

alert(external); // Prints 5
firsttry(); // Prints 6
alert(external); // Prints 5
secondtry(); // Prints 7
alert(external); // Prints 7
</code></pre>

The second function alters the value of the global variable "external", but the first function doesn't.

blocks|key|1082795|text|Javascript中的var关键字是有目的的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1082796|如果您声明一个不带var关键字的变量，如下所示：|1082797|myVar+=+100;|code-block|syntax|javascript|1082798|它成为一个全局变量，可以从脚本的任何部分进行访问。如果您不是故意这样做的，或者没有意识到这一点，那么在javascript中的另一个地方重复使用变量名可能会让您痛苦不已。|1082799|如果使用var关键字声明变量，如下所示：|1082800|var+myVar+=+100;|1082801|它是作用域的本地({]+-大括号，函数，文件，取决于你放置它的位置)。|1082802|这是一种更安全的处理变量的方法。因此，除非您是故意这样做的，否则请尝试使用var关键字声明变量，而不是不使用。|1082803|entityMap^0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|V|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|W|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|X|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|Y|8|@]|9|@]|A|$]]|$1|M|3|N|5|F|7|Z|8|@]|9|@]|A|$G|H]]|$1|O|3|P|5|6|7|10|8|@]|9|@]|A|$]]|$1|Q|3|R|5|6|7|11|8|@]|9|@]|A|$]]|$1|S|3|-4|5|6|7|12|8|@]|9|@]|A|$]]]|T|$]]

The var keyword in Javascript is there for a purpose.

If you declare a variable without the var keyword, like this:

<pre><code>myVar = 100;
</code></pre>

It becomes a global variable that can be accessed from any part of your script. If you did not do it intentionally or are not aware of it, it can cause you pain if you re-use the variable name at another place in your javascript.

If you declare the variable with the var keyword, like this:

<pre><code>var myVar = 100;
</code></pre>

It is local to the scope ({] - braces, function, file, depending on where you placed it).

This a safer way to treat variables. So unless you are doing it on purpose try to declare variable with the var keyword and not without.

blocks|key|1082864|text|围绕这个主题有太多的困惑，而且现有的答案中没有一个清楚而直接地涵盖了一切。下面是一些带有内联注释的示例。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1082865|//this+is+a+declaration
var+foo;

//this+is+an+assignment
bar+=+3;

//this+is+a+declaration+and+an+assignment
var+dual+=+5;|code-block|syntax|javascript|1082866|声明设置DontDelete标志。赋值则不会。|1082867|声明将该变量绑定到当前作用域。|1082868|已分配但未声明的变量将查找要附加其自身的作用域。这意味着它将遍历作用域的食物链，直到找到具有相同名称的变量。如果没有找到，它将被附加到顶级作用域(通常称为全局作用域)。|1082869|function+example(){
++//is+a+member+of+the+scope+defined+by+the+function+example
++var+foo;

++//this+function+is+also+part+of+the+scope+of+the+function+example
++var+bar+=+function(){
+++++foo+=+12;+//+traverses+scope+and+assigns+example.foo+to+12
++}
}

function+something_different(){
+++++foo+=+15;+//+traverses+scope+and+assigns+global.foo+to+15
}|1082870|为了清楚地描述正在发生的事情，此analysis+of+the+delete+function广泛地介绍了变量实例化和赋值。|offset|length|1082871|entityMap|0|LINK|mutability|MUTABLE|url|http://perfectionkills.com/understanding-delete/^0|0|0|0|0|0|0|G|V|0|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]]|$1|G|3|H|5|6|7|12|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|13|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|14|8|@]|9|@]|A|$]]|$1|M|3|N|5|D|7|15|8|@]|9|@]|A|$E|F]]|$1|O|3|P|5|6|7|16|8|@]|9|@$Q|17|R|18|1|19]]|A|$]]|$1|S|3|-4|5|6|7|1A|8|@]|9|@]|A|$]]]|T|$U|$5|V|W|X|A|$Y|Z]]]]

There's so much confusion around this subject, and none of the existing answers cover everything clearly and directly. Here are some examples with comments inline.

<pre><code>//this is a declaration
var foo;

//this is an assignment
bar = 3;

//this is a declaration and an assignment
var dual = 5;
</code></pre>

A declaration sets a DontDelete flag. An assignment does not.

A declaration ties that variable to the current scope.

A variable assigned but not declared will look for a scope to attach itself to. That means it will traverse up the food-chain of scope until a variable with the same name is found. If none is found, it will be attached to the top-level scope (which is commonly referred to as global).

<pre><code>function example(){
 //is a member of the scope defined by the function example
 var foo;

 //this function is also part of the scope of the function example
 var bar = function(){
 foo = 12; // traverses scope and assigns example.foo to 12
 }
}

function something_different(){
 foo = 15; // traverses scope and assigns global.foo to 15
}
</code></pre>

For a very clear description of what is happening, this <a href="http://perfectionkills.com/understanding-delete/">analysis of the delete function</a> covers variable instantiation and assignment extensively.

blocks|key|1297983|text|看看这个提琴：http://jsfiddle.net/GWr6Z/2/|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1297984|function+doMe(){

a+=+"123";+//+will+be+global
var+b+=+"321";+//+local+to+doMe
alert("a:"%2Ba%2B"++--++b:"%2Bb);

b+=+"something+else";+//+still+local+(not+global)
alert("a:"%2Ba%2B"++--++b:"%2Bb);

};


doMe()
alert("a:"%2Ba%2B"++--++b:"%2Bb);+//+`b`+will+not+be+defined,+check+console.log|code-block|syntax|javascript|1297985|entityMap|0|LINK|mutability|MUTABLE|url|http://jsfiddle.net/GWr6Z/2/^0|7|S|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@$A|R|B|S|1|T]]|C|$]]|$1|D|3|E|5|F|7|U|8|@]|9|@]|C|$G|H]]|$1|I|3|-4|5|6|7|V|8|@]|9|@]|C|$]]]|J|$K|$5|L|M|N|C|$O|P]]]]

Check out this Fiddle: <a href="http://jsfiddle.net/GWr6Z/2/" rel="noreferrer">http://jsfiddle.net/GWr6Z/2/</a>

<pre><code>function doMe(){

a = "123"; // will be global
var b = "321"; // local to doMe
alert("a:"+a+" -- b:"+b);

b = "something else"; // still local (not global)
alert("a:"+a+" -- b:"+b);

};


doMe()
alert("a:"+a+" -- b:"+b); // `b` will not be defined, check console.log
</code></pre>

blocks|key|1082928|text|我刚刚在我的一个同事推荐的论坛上找到了答案。如果你在函数外部声明一个变量，它总是全局的。不管您是否使用var关键字。但是，如果你在一个函数中声明变量，它就会有很大的不同。在函数内部，如果使用var关键字声明变量，它将是局部的，但如果不使用var关键字声明变量，它将是全局的。它可以覆盖你之前声明的变量。-更多信息请访问：http://forum.webdeveloperszone.com/question/what-is-the-difference-between-using-var-keyword-or-not-using-var-during-variable-declaration/#sthash.xNnLrwc3.dpuf|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1082929|entityMap|0|LINK|mutability|MUTABLE|url|http://forum.webdeveloperszone.com/question/what-is-the-difference-between-using-var-keyword-or-not-using-var-during-variable-declaration/#sthash.xNnLrwc3.dpuf^0|4G|4F|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@$A|M|B|N|1|O]]|C|$]]|$1|D|3|-4|5|6|7|P|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]]]

I just found the answer from a forum referred by one of my colleague. If you declare a variable outside a function, it's always global. No matter if you use var keyword or not. But, if you declare the variable inside a function, it has a big difference. Inside a function, if you declare the variable using var keyword, it will be local, but if you declare the variable without var keyword, it will be global. It can overwrite your previously declared variables. - See more at: <a href="http://forum.webdeveloperszone.com/question/what-is-the-difference-between-using-var-keyword-or-not-using-var-during-variable-declaration/#sthash.xNnLrwc3.dpuf" rel="nofollow">http://forum.webdeveloperszone.com/question/what-is-the-difference-between-using-var-keyword-or-not-using-var-during-variable-declaration/#sthash.xNnLrwc3.dpuf</a>

blocks|key|1298054|text|抛开作用域不谈，它们可以用不同的方式使用。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1298055|console.out(var+myObj=1);
//SyntaxError:+Unexpected+token+var
console.out(myObj=1);
//1|code-block|syntax|javascript|1298056|陈述vs表达式|1298057|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

Everything about scope aside, they can be used differently. 

<pre><code>console.out(var myObj=1);
//SyntaxError: Unexpected token var
console.out(myObj=1);
//1
</code></pre>

Something something statement vs expression

blocks|key|1083031|text|不，它不是“必需的”，但如果你不这样做，它可能会导致严重的问题。不使用var定义变量会将该变量放在它所在的代码部分的作用域内。如果不这样做，那么它就不会包含在该作用域中，并且可以覆盖先前定义的同名变量，这些变量位于您所在的函数的作用域之外。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1083032|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

No, it is not "required", but it might as well be as it can cause major issues down the line if you don't. Not defining a variable with var put that variable inside the scope of the part of the code it's in. If you don't then it isn't contained in that scope and can overwrite previously defined variables with the same name that are outside the scope of the function you are in.

Is "var" optional?

<pre><code>myObj = 1;
</code></pre>

same as ?

<pre><code>var myObj = 1;
</code></pre>

I found they both work from my test, I assume <code>var</code> is optional. Is that right?

Is using 'var' to declare variables optional?

"var“是可选的吗？myObj = 1;跟什么一样？var myObj = 1;我在测试中发现它们都可以工作，我认为var是可选的。是那么回事吗？

问使用'var‘声明变量是可选的吗？
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问使用'var‘声明变量是可选的吗？
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