blocks|key|1427786|text|检查jQuery+FAQ...|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1427787|可以使用选择器返回的jQuery集合的length属性：|1427788|if+(+$('#myDiv').length+){}|code-block|syntax|javascript|1427789|entityMap|0|LINK|mutability|MUTABLE|url|http://learn.jquery.com/using-jquery-core/faq/how-do-i-test-whether-an-element-exists/^0|2|A|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@$A|T|B|U|1|V]]|C|$]]|$1|D|3|E|5|6|7|W|8|@]|9|@]|C|$]]|$1|F|3|G|5|H|7|X|8|@]|9|@]|C|$I|J]]|$1|K|3|-4|5|6|7|Y|8|@]|9|@]|C|$]]]|L|$M|$5|N|O|P|C|$Q|R]]]]

Check the <a href="http://learn.jquery.com/using-jquery-core/faq/how-do-i-test-whether-an-element-exists/" rel="noreferrer">jQuery FAQ</a>...

You can use the length property of the jQuery collection returned by your selector:

<pre><code>if ( $('#myDiv').length ){}
</code></pre>

blocks|key|1427738|text|lookup函数返回匹配元素的数组。您可以检查长度是否为零。请注意，更改为只查找元素一次，并根据需要重用结果。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1427739|var+elem+=+$("#btext"+%2B+i);
if+(elem.length+!=+0)+{
+++elem.text("Branch+"+%2B+i);
}|code-block|syntax|javascript|1427740|另外，您是否尝试过仅使用text函数--如果不存在任何元素，它将不会执行任何操作。|1427741|$("#btext"+%2B+i).text("Branch+"+%2B+i);|1427742|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

The lookup function returns an array of matching elements. You could check if the length is zero. Note the change to only look up the elements once and reuse the results as needed.

<pre><code>var elem = $("#btext" + i);
if (elem.length != 0) {
 elem.text("Branch " + i);
}
</code></pre>

Also, have you tried just using the text function -- if no element exists, it will do nothing.

<pre><code>$("#btext" + i).text("Branch " + i);
</code></pre>

blocks|key|1642715|text|jquery+$()函数总是返回非空值-与选择器cretaria匹配的均值元素。如果没有找到该元素，它将返回一个空数组。所以你的代码看起来会像这样-|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1642716|if+($("#btext"+%2B+i).length){
++++++++//alert($("#btext"+%2B+i).text());
++++$("#btext"+%2B+i).text("Branch+"+%2B+i);
}|code-block|syntax|javascript|1642717|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|$]]

jquery $() function always return non null value - mean elements matched you selector cretaria. If the element was not found it will return an empty array. 
So your code will look something like this - 

<pre><code>if ($("#btext" + i).length){
 //alert($("#btext" + i).text());
 $("#btext" + i).text("Branch " + i);
}
</code></pre>

blocks|key|1427992|text|无论您选择的是哪个函数，$()都会返回一个jQuery对象，因此不能用于测试。最好的方法(如果不是唯一的方法)是使用size()函数或如上所述的原生长度属性。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1427993|if+(+$('selector').size()+)+{...}+++++++++++++++++++|code-block|syntax|javascript|1427994|entityMap^0|C|3|1M|6|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|$]]

no matter what you selection is the function <code>$()</code> always returns a jQuery object so that cant be used to test. The best way yet (if not the only) is to use the <code>size()</code> function or the native length property as explained above.

<pre><code>if ( $('selector').size() ) {...} 
</code></pre>

blocks|key|1642993|text|在Drupal1.4中，您可以使用$.isEmptyObject函数，但是如果您被迫使用旧版本的jQ，就像我们这些可怜的jQuery开发人员一样，只需盗用以下代码：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1642994|//+This+is+a+function+similar+to+the+jQuery+1.4+$.isEmptyObject.
function+isObjectEmpty(obj)+{
++for+(+var+name+in+obj+)+{
++++return+false;
++}
++return+true;
}|code-block|syntax|javascript|1642995|像这样使用它：|1642996|console.log(isObjectEmpty(the_object));+//+Returns+true+or+false.|1642997|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

In jQuery 1.4 you get the $.isEmptyObject function, but if you are forced to use an older version of jQ like us poor Drupal developers just steal use this code:

<pre><code>// This is a function similar to the jQuery 1.4 $.isEmptyObject.
function isObjectEmpty(obj) {
 for ( var name in obj ) {
 return false;
 }
 return true;
}
</code></pre>

Use it like:

<pre><code>console.log(isObjectEmpty(the_object)); // Returns true or false.
</code></pre>

blocks|key|1643091|text|如果使用"undefined“呢？|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1643092|if+(value+!=+undefined){+//+do+stuff+}+|code-block|syntax|javascript|1643093|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|$]]

What about using "undefined"?

<pre><code>if (value != undefined){ // do stuff } 
</code></pre>

blocks|key|1427911|text|if+(+$('#whatever')[0]+)+{...}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|1427912|由所有原生jQuery方法返回的jQuery对象不是一个数组，它是一个具有许多属性的对象；其中一个属性是"length“属性。您还可以检查size()或get(0)或get()+-+'get(0)‘的工作方式与访问第一个元素相同，即$(elem)|unstyled|1427913|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 ( $('#whatever')[0] ) {...}
</code></pre>

The jQuery object which is returned by all native jQuery methods is NOT an array, it is an object with many properties; one of them being a "length" property. You can also check for size() or get(0) or get() - 'get(0)' works the same as accessing the first element, i.e. $(elem)[0]

blocks|key|1642929|text|使用$("#selector").get(0)像这样检查null。get返回dom元素，在此之前，您要处理的是一个数组，其中需要检查length属性。我个人不喜欢空值处理的长度检查，因为某些原因它让我感到困惑:)|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1642930|entityMap^0|2|L|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 <code>$("#selector").get(0)</code> to check with null like that. get returns the dom element, until then you re dealing with an array, where you need to check the length property. I personally don't like length check for null handling, it confuses me for some reason :)

blocks|key|1428207|text|使用length属性可以做到这一点。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1428208|jQuery.fn.exists+=+function(){return+($(this).length+<+0);}
if+($(selector).exists())+{+
+++//do+somthing
}|code-block|syntax|javascript|1428209|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|$]]

Using the length property you can do this.

<pre><code>jQuery.fn.exists = function(){return ($(this).length &lt; 0);}
if ($(selector).exists()) { 
 //do somthing
}
</code></pre>

blocks|key|1643064|text|当对象为空时，返回以下错误：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1643065|Uncaught+TypeError:+Cannot+read+property+'0'+of+null|code-block|syntax|javascript|1643066|我尝试了下面的代码：|1643067|try{
++if+($("#btext"+%2B+i).length)+{};+++++++++++++++
}catch(err){
++if+($("#btext"+%2B+i).length)+{
+++++//working+this+code+if+the+item,+not+NULL+
+++}
}|1643068|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

when the object is empty return this error:

<pre><code>Uncaught TypeError: Cannot read property '0' of null
</code></pre>

I try this code :

<pre><code>try{
 if ($("#btext" + i).length) {}; 
}catch(err){
 if ($("#btext" + i).length) {
 //working this code if the item, not NULL 
 }
}
</code></pre>

blocks|key|1642796|text|if+(typeof($("#btext"+%2B+i))+==+'object'){
++++$("#btext"+%2B+i).text("Branch+"+%2B+i);
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|1642797|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 (typeof($("#btext" + i)) == 'object'){
 $("#btext" + i).text("Branch " + i);
}
</code></pre>

blocks|key|1642873|text|调用未定义或空对象的长度属性将导致IE和webkit浏览器失败！|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1642874|相反，尝试这样做：|1642875|if($("#something")+!==+null){
++//+do+something
}|code-block|syntax|javascript|1642876|或|1642877|if($("#something")+===+null){
++//+don't+do+something
}|1642878|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|P|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|Q|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|R|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|S|8|@]|9|@]|A|$G|H]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

Calling length property on undefined or a null object will cause IE and webkit browsers to fail!

Instead try this:

<pre><code>if($("#something") !== null){
 // do something
}
</code></pre>

or

<pre><code>if($("#something") === null){
 // don't do something
}
</code></pre>

I'm using jQuery and I want to check the existence of an element in my page. I have written following code, but it's not working:

<pre><code>if($("#btext" + i) != null) {
 //alert($("#btext" + i).text());
 $("#btext" + i).text("Branch " + i);
}
</code></pre>

How do I check the existence of the element?

How to check null objects in jQuery

我使用的是jQuery，我想检查页面中是否存在元素。我写了以下代码，但它不工作：if($("#btext" + i) != null) {    //alert($("#btext" + i).text());    $("#btext" + i).text("Branch " + i);}如何检查元素的存在？

问如何在jQuery中检查空对象
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问如何在jQuery中检查空对象
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