blocks|key|228152|text|JQuery库是专门为简化和统一某些JavaScript功能而开发的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|228153|但是，如果您需要根据undefined值检查变量，则不需要发明任何特殊方法，因为JavaScript有一个typeof运算符，它简单、快速且跨平台：|offset|length|style|CODE|228154|if+(typeof+value+===+"undefined")+{
++++//+...
}|code-block|syntax|javascript|228155|它返回一个字符串，指示变量或其他未计算的操作数的类型。与if+(value+===+undefined)+{+...+}相比，这种方法的主要优点是，如果变量value不存在，typeof永远不会引发异常。|228156|entityMap|0|LINK|mutability|MUTABLE|url|https://developer.mozilla.org/en/JavaScript/Reference/Operators/typeof^0|0|A|9|1H|6|1H|6|0|0|0|S|W|26|5|2F|6|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|X|8|@$D|Y|E|Z|F|G]|$D|10|E|11|F|G]]|9|@$D|12|E|13|1|14]]|A|$]]|$1|H|3|I|5|J|7|15|8|@]|9|@]|A|$K|L]]|$1|M|3|N|5|6|7|16|8|@$D|17|E|18|F|G]|$D|19|E|1A|F|G]|$D|1B|E|1C|F|G]]|9|@]|A|$]]|$1|O|3|-4|5|6|7|1D|8|@]|9|@]|A|$]]]|P|$Q|$5|R|S|T|A|$U|V]]]]

JQuery library was developed specifically to simplify and to unify certain JavaScript functionality.

However if you need to check a variable against <code>undefined</code> value, there is no need to invent any special method, since JavaScript has a <a href="https://developer.mozilla.org/en/JavaScript/Reference/Operators/typeof"><code>typeof</code></a> operator, which is simple, fast and cross-platform:

<pre><code>if (typeof value === "undefined") {
 // ...
}
</code></pre>

It returns a string indicating the type of the variable or other unevaluated operand. The main advantage of this method, compared to <code>if (value === undefined) { ... }</code>, is that <code>typeof</code> will never raise an exception in case if variable <code>value</code> does not exist.

blocks|key|229800|text|请注意，typeof总是返回一个字符串，如果变量根本不存在，则不会生成错误。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|229801|function+A(val){
++if(typeof(val)++===+"undefined")+
++++//do+this
++else
+++//do+this
}|code-block|syntax|javascript|229802|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Note that typeof always returns a string, and doesn't generate an error if the variable doesn't exist at all.

<pre><code>function A(val){
 if(typeof(val) === "undefined") 
 //do this
 else
 //do this
}
</code></pre>

blocks|key|227507|text|在这种情况下，您可以使用===+undefined比较：if(val+===+undefined)|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|227508|这之所以有效，是因为val始终存在(它是一个函数参数)。|227509|如果你想测试一个不是参数的任意变量，也就是说可能根本没有定义，你必须使用if(typeof+val+===+'undefined')来避免val不存在的异常。|227510|entityMap^0|C|D|S|L|0|A|3|0|10|U|1X|3|0^^$0|@$1|2|3|4|5|6|7|L|8|@$9|M|A|N|B|C]|$9|O|A|P|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|Q|8|@$9|R|A|S|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|T|8|@$9|U|A|V|B|C]|$9|W|A|X|B|C]]|D|@]|E|$]]|$1|J|3|-4|5|6|7|Y|8|@]|D|@]|E|$]]]|K|$]]

In this case you can use a <code>=== undefined</code> comparison: <code>if(val === undefined)</code>

This works because <code>val</code> always exists (it's a function argument).

If you wanted to test an arbitrary variable that is not an argument, i.e. might not be defined at all, you'd have to use <code>if(typeof val === 'undefined')</code> to avoid an exception in case <code>val</code> didn't exist.

blocks|key|3096921|text|您可以使用速记技术检查它是未定义的还是空的|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3096922|+function+A(val)
+{
+++if(val+%7C%7C+"")+
+++//do+this
+else
+//do+this
+}|code-block|syntax|javascript|3096923|希望这能对你有所帮助|3096924|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|$]]

You can use shorthand technique to check whether it is undefined or null 

<pre><code> function A(val)
 {
 if(val || "") 
 //do this
 else
 //do this
 }
</code></pre>

hope this will help you

blocks|key|229856|text|我知道我回答这个函数来晚了，但是jquery有一个内置函数来做这件事|type|unstyled|depth|inlineStyleRanges|entityRanges|data|229857|if(jQuery.type(val)+===+"undefined"){
++++//Some+code+goes+here
}|code-block|syntax|javascript|229858|请参考jquery.type+https://api.jquery.com/jQuery.type/的jquery+API文档。|offset|length|229859|entityMap|0|LINK|mutability|MUTABLE|url|https://api.jquery.com/jQuery.type/^0|0|0|F|Z|0|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]]|$1|G|3|H|5|6|7|U|8|@]|9|@$I|V|J|W|1|X]]|A|$]]|$1|K|3|-4|5|6|7|Y|8|@]|9|@]|A|$]]]|L|$M|$5|N|O|P|A|$Q|R]]]]

I know I am late to answer the function but jquery have a in build function to do this 

<pre><code>if(jQuery.type(val) === "undefined"){
 //Some code goes here
}
</code></pre>

Refer jquery API document of jquery.type <a href="https://api.jquery.com/jQuery.type/" rel="noreferrer">https://api.jquery.com/jQuery.type/</a> for the same.

blocks|key|228204|text|如果您有元素的名称而不是id，我们可以实现对所有文本元素(例如)的未定义检查，如下所示，并使用默认值填充它们，例如0.0：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|228205|var+aFieldsCannotBeNull=['ast_chkacc_bwr','ast_savacc_bwr'];
+jQuery.each(aFieldsCannotBeNull,function(nShowIndex,sShowKey)+{
+++var+$_oField+=+jQuery("input[name='"%2BsShowKey%2B"']");
+++if($_oField.val().trim().length+===+0){
+++++++$_oField.val('0.0')
++++}
++})|code-block|syntax|javascript|228206|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

If you have names of the element and not id we can achieve the undefined check on all text elements (for example) as below and fill them with a default value say 0.0:

<pre><code>var aFieldsCannotBeNull=['ast_chkacc_bwr','ast_savacc_bwr'];
 jQuery.each(aFieldsCannotBeNull,function(nShowIndex,sShowKey) {
 var $_oField = jQuery("input[name='"+sShowKey+"']");
 if($_oField.val().trim().length === 0){
 $_oField.val('0.0')
 }
 })
</code></pre>

blocks|key|3097023|text|当我测试"typeof+obj+===+undefined“时，alert(typeof+obj)返回object，即使obj是未定义的。因为obj的类型是Object，它返回的是Object，而不是undefined。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3097024|因此，经过几个小时的测试后，我选择了下面的技术。|3097025|if(document.getElementById(obj)+!==+null){
//do...
}else{
//do...
}|code-block|syntax|javascript|3097026|我不确定为什么第一个技术没有work.But我用这个完成了我的工作。|3097027|entityMap^0|5|O|W|H|1F|6|26|6|2I|6|2S|9|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@$9|R|A|S|B|C]|$9|T|A|U|B|C]|$9|V|A|W|B|C]|$9|X|A|Y|B|C]|$9|Z|A|10|B|C]|$9|11|A|12|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|13|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|14|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|15|8|@]|D|@]|E|$]]|$1|O|3|-4|5|6|7|16|8|@]|D|@]|E|$]]]|P|$]]

when I am testing "<code>typeof obj === undefined</code>", the <code>alert(typeof obj)</code> returning <code>object</code>, even though obj is undefined.
Since obj is type of <code>Object</code> its returning <code>Object</code>, not <code>undefined</code>.

So after hours of testing I opted below technique.

<pre><code>if(document.getElementById(obj) !== null){
//do...
}else{
//do...
}
</code></pre>

I am not sure why the first technique didn't work.But I get done my work using this.

blocks|key|3097037|text|我不确定这是不是最好的解决方案，但它工作得很好：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3097038|if($someObject['length']!=0){
++++//do+someting
}|code-block|syntax|javascript|3097039|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

I am not sure it is the best solution, but it works fine:

<pre><code>if($someObject['length']!=0){
 //do someting
}
</code></pre>

blocks|key|229878|text|function+isValue(value,+def,+is_return)+{
++++if+(+$.type(value)+==+'null'
++++++++%7C%7C+$.type(value)+==+'undefined'
++++++++%7C%7C+$.trim(value)+==+''
++++++++%7C%7C+($.type(value)+==+'number'+&&+!$.isNumeric(value))
++++++++%7C%7C+($.type(value)+==+'array'+&&+value.length+==+0)
++++++++%7C%7C+($.type(value)+==+'object'+&&+$.isEmptyObject(value))+)+{
++++++++return+($.type(def)+!=+'undefined')+?+def+:+false;
++++}+else+{
++++++++return+($.type(is_return)+==+'boolean'+&&+is_return+===+true+?+value+:+true);
++++}
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|229879|尝试此~所有类型检查器|unstyled|229880|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>function isValue(value, def, is_return) {
 if ( $.type(value) == 'null'
 || $.type(value) == 'undefined'
 || $.trim(value) == ''
 || ($.type(value) == 'number' &amp;&amp; !$.isNumeric(value))
 || ($.type(value) == 'array' &amp;&amp; value.length == 0)
 || ($.type(value) == 'object' &amp;&amp; $.isEmptyObject(value)) ) {
 return ($.type(def) != 'undefined') ? def : false;
 } else {
 return ($.type(is_return) == 'boolean' &amp;&amp; is_return === true ? value : true);
 }
}
</code></pre>

try this~ all type checker

blocks|key|224637|text|if+(value+===+undefined)+{
++++//+...
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|224638|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>if (value === undefined) {
 // ...
}
</code></pre>

blocks|key|2883041|text|您可以使用两种方式1)+if+(+val+==+null+)+2)+if+(+val+===+undefine+)|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2883042|entityMap^0|C|I|Y|N|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|M|8|@]|D|@]|E|$]]]|G|$]]

You can use two way
1) <code>if ( val == null )</code>
2) <code>if ( val === undefine )</code>

<blockquote>
 Possible Duplicate: 
 <a href="https://stackoverflow.com/questions/27509/detecting-an-undefined-object-property-in-javascript">Detecting an undefined object property in JavaScript</a> 
 <a href="https://stackoverflow.com/questions/2778901/javascript-undefined-compare">javascript undefined compare</a> 
</blockquote>



How we can add a check for an undefined variable, like:

<pre><code>function A(val) {
 if (val == undefined) 
 // do this
 else
 // do this
}
</code></pre>

How to check 'undefined' value in jQuery

jQuery

 可能重复：如何添加对未定义变量的检查，例如：function A(val) {  if (val == undefined)     // do this  else    // do this}

问如何在jQuery中检查“未定义”值
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