blocks|key|5286749|text|与typeof一起使用时，'undefined'需要用引号括起来|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|5286750|if(typeof+ticketType+!=+'undefined'+&&+ticketType+==+1){}|code-block|syntax|javascript|5286751|entityMap^0|1|6|D|B|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]|$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|R|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|S|8|@]|D|@]|E|$]]]|L|$]]

<code>'undefined'</code> needs to have quotes around it when used with <code>typeof</code>

<pre><code>if(typeof ticketType != 'undefined' && ticketType == 1){}</code></pre>

blocks|key|5002182|text|if(typeof+ticketType+!=+'undefined'+&&+ticketType+==+1){}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|5002183|我认为你需要在未定义的单词周围加上引号。至少我总是这样做。|unstyled|5002184|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|J|8|@]|9|@]|A|$]]|$1|G|3|-4|5|F|7|K|8|@]|9|@]|A|$]]]|H|$]]

<pre><code>if(typeof ticketType != 'undefined' && ticketType == 1){}</code></pre>

I think you need the quotes around the undefined word. At least I always do it that way.

blocks|key|5286786|text|用括号把它括起来。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|5286787|if+(typeof(ticketType)+!==+'undefined'+&&+ticketType+===+1)|offset|length|style|CODE|5286788|entityMap^0|0|0|1N|0^^$0|@$1|2|3|4|5|6|7|J|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|K|8|@$D|L|E|M|F|G]]|9|@]|A|$]]|$1|H|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|I|$]]

Wrap it in parenthesis.

<code>if (typeof(ticketType) !== 'undefined' &amp;&amp; ticketType === 1)</code>

blocks|key|5002222|text|正确的语法是：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|5002223|if+(typeof+ticketType+!==+'undefined'+&&+ticketType+===+1)+{+}|code-block|syntax|javascript|5002224|typeof运算符的结果始终是字符串。查看此处：http://www.javascriptkit.com/javatutors/determinevar2.shtml|offset|length|style|CODE|5002225|entityMap|0|LINK|mutability|MUTABLE|url|http://www.javascriptkit.com/javatutors/determinevar2.shtml^0|0|0|0|6|O|1N|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|W|8|@$I|X|J|Y|K|L]]|9|@$I|Z|J|10|1|11]]|A|$]]|$1|M|3|-4|5|6|7|12|8|@]|9|@]|A|$]]]|N|$O|$5|P|Q|R|A|$S|T]]]]

The correct syntax is:

<pre><code>if (typeof ticketType !== 'undefined' &amp;&amp; ticketType === 1) { }
</code></pre>

The result of the <code>typeof</code> operator is always a string. See here: <a href="http://www.javascriptkit.com/javatutors/determinevar2.shtml" rel="nofollow">http://www.javascriptkit.com/javatutors/determinevar2.shtml</a>

blocks|key|3388638|text|错误消息非常清楚。试试这个：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3388639|var+ticketType;
if(typeof+ticketType+!=+undefined+&&+ticketType+==+1){}|code-block|syntax|javascript|3388640|您不能只引用不存在的变量。您只能在分配时执行此操作：|3388641|ticketType+=+1;|3388642|浏览器抱怨是因为ticketType是一个未知的标识符。但是，如果我们首先声明它(即使没有给它赋任何值)，它就会变得已知，但带有undefined值。|offset|length|style|CODE|3388643|entityMap^0|0|0|0|0|8|A|1S|9|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|U|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|W|8|@$M|X|N|Y|O|P]|$M|Z|N|10|O|P]]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|11|8|@]|9|@]|A|$]]]|R|$]]

The error message is pretty clear. Try with this:

<pre><code>var ticketType;
if(typeof ticketType != undefined &amp;&amp; ticketType == 1){}
</code></pre>

You cannot just reference a variable that does not exist. You can only do this when assigning:

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

The browser complaints because <code>ticketType</code> is an unknown identifier. However if we first declare it (even without assigning it any value) it becomes known but with <code>undefined</code> value.

blocks|key|5002293|text|未定义应包含在引号内...|type|unstyled|depth|inlineStyleRanges|entityRanges|data|5002294|if+(typeof+ticketType+!==+"undefined"+&&+ticketType+==+1)
{
}|code-block|syntax|javascript|5002295|编辑|offset|length|style|BOLD|5002296|这里我们不是在讨论global.undefined，它不需要用引号括起来。我们讨论的是typeof+operator的返回类型，它是一个字符串。顺便说一句，对于未定义的变量，typeof返回"+undefined“，因此我们需要将它包含在string中。|5002297|//+ticketType+is+not+defined+yet

(typeof+ticketType+!==+undefined)+//+This+is+true
(typeof+ticketType+===+undefined)+//+This+is+false
(typeof+ticketType+!==+"undefined")+//+This+is+false
(typeof+ticketType+===+"undefined")+//+This+is+true

var+ticketType+=+"someValue";+//+ticketType+is+defined

(typeof+ticketType+!==+undefined)+//+This+is+still+true
(typeof+ticketType+===+undefined)+//+This+is+still+false
(typeof+ticketType+!==+"undefined")+//+This+is+true
(typeof+ticketType+===+"undefined")+//+This+is+false|5002298|因此，正确的检查是针对"undefined"而不是针对global.undefined。|CODE|5002299|entityMap|0|LINK|mutability|MUTABLE|url|https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/undefined|1|https://developer.mozilla.org/en/JavaScript/Reference/Operators/typeof^0|0|0|0|2|0|9|G|0|17|F|1|0|0|B|B|R|G|0^^$0|@$1|2|3|4|5|6|7|13|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|14|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|15|8|@$I|16|J|17|K|L]]|9|@]|A|$]]|$1|M|3|N|5|6|7|18|8|@]|9|@$I|19|J|1A|1|1B]|$I|1C|J|1D|1|1E]]|A|$]]|$1|O|3|P|5|D|7|1F|8|@]|9|@]|A|$E|F]]|$1|Q|3|R|5|6|7|1G|8|@$I|1H|J|1I|K|S]|$I|1J|J|1K|K|S]]|9|@]|A|$]]|$1|T|3|-4|5|6|7|1L|8|@]|9|@]|A|$]]]|U|$V|$5|W|X|Y|A|$Z|10]]|11|$5|W|X|Y|A|$Z|12]]]]

undefined should be within quotes...

<pre><code>if (typeof ticketType !== "undefined" &amp;&amp; ticketType == 1)
{
}
</code></pre>

EDIT

Here we are not talking about <a href="https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/undefined" rel="nofollow">global.undefined</a> which doesn't have to be enclosed within quotes. We are talking about the return type of <a href="https://developer.mozilla.org/en/JavaScript/Reference/Operators/typeof" rel="nofollow">typeof operator</a> which is a string. Incidentally for undefined variable, the typeof returns "undefined" and thus we need to enclose it within string.

<pre><code>// ticketType is not defined yet

(typeof ticketType !== undefined) // This is true
(typeof ticketType === undefined) // This is false
(typeof ticketType !== "undefined") // This is false
(typeof ticketType === "undefined") // This is true

var ticketType = "someValue"; // ticketType is defined

(typeof ticketType !== undefined) // This is still true
(typeof ticketType === undefined) // This is still false
(typeof ticketType !== "undefined") // This is true
(typeof ticketType === "undefined") // This is false
</code></pre>

So the correct check is against <code>"undefined"</code> not against <code>global.undefined</code>.

blocks|key|5002342|text|你错误地检查了一个未定义的变量，但是如果你打算使用它，为什么不确保它被定义呢？|type|unstyled|depth|inlineStyleRanges|entityRanges|data|5002343|ticketType+=+ticketType+%7C%7C+1;
if+(ticketType+===+1)+{
++//do+stuff
}|code-block|syntax|javascript|5002344|5002345|正如其他人所展示的，检查JS中未定义的值的标准方法是：|5002346|typeof+somevar+===+'undefined'|5002347|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|O|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|P|8|@]|9|@]|A|$]]|$1|H|3|I|5|6|7|Q|8|@]|9|@]|A|$]]|$1|J|3|K|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|L|3|-4|5|6|7|S|8|@]|9|@]|A|$]]]|M|$]]

you're incorrectly checking for an undefined variable, but if you're planning on using it, why not just make sure that it gets defined?

<pre><code>ticketType = ticketType || 1;
if (ticketType === 1) {
 //do stuff
}
</code></pre>

<hr>

As everyone else has shown, the standard way of checking for undefined values in JS is:

<pre><code>typeof somevar === 'undefined'
</code></pre>

blocks|key|5286947|text|简单使用console.log(someVar)；|type|unstyled|depth|inlineStyleRanges|entityRanges|data|5286948|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|$]]

simlpe using console.log(someVar);

I have three pages utilizing the same code and on one of the pages this variable doesn't exist, on the other two the variable ticketType has a value of 1 or 2. I need to first check if ticketType exists and isn't undefined and secondly, need to determine if it's one or 2.

This if statement generates an error:

<pre><code>if(typeof ticketType != undefined &amp;&amp; ticketType == 1){}
</code></pre>

It's saying <code>ticketType isn't defined</code>. I tried nesting the if statements to check if it was defined first thinking it wouldn't go and try the inner if statement but firebug still generates an error.

Any ideas? There has to be a way to do this...

JavaScript : Check if variable exists and if equal to value

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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我有三个页面使用了相同的代码，其中一个页面上的变量不存在，另外两个页面上的变量ticketType的值是1或2。我需要首先检查ticketType是否存在以及是否未定义，然后需要确定它是1还是2。下面的if语句生成一个错误：if(typeof ticketType != undefined && ticketType ...

问JavaScript :检查变量是否存在以及是否等于值
EN

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问JavaScript :检查变量是否存在以及是否等于值EN